Answering your title question: It depends—on your own memory; on whether you have continued to play it (and how often); how complex it is. … I know there are tunes I used to play that I could not grab my banjo and play now. I could get them back though, with a lot less work than learning a new one from scratch. That doesn’t apply though, for a tune I learned and only played a time or two. Those are pretty much gone. For that matter, I know there are tunes I used to play that I don’t even remember the names of, let alone how to play them.

“How do i compare to other people in this regard?“ Normal.

Songs are like old friends. They bring comfort, or a smile, or a tear. They speak honestly to you. You get to know them very well. And as the years go by, you sometimes forget their name, or their birthday, or whatever.
With some friends you can pick up where you left off. With others, it takes a little time to “rekindle the romance.”

And sometimes you’ll start on a song that feels so familiar, but you would swear that …”Haven’t we met somewhere before?”

I think you’re Normal, with many, many friends, including old Whatshisname.

I'm a little older than the OP, and sadly I too find that I forget how to play things over time. Most of the pieces I learn are learnt by ear rather than from tab, partly because I can't often find tabs for the more complex pieces and partly because I enjoy the 'by ear' process. Once having learnt the tune to my satisfaction, I tend to move on to something else. After a (sometimes worryingly short) time, I find that I can't just play the piece I've moved on from, and it takes a while to re-establish it in my subconscious. I think this is fairly normal for a player in his mid-seventies, especially if the tunes in question are of a complex nature. However, there are some tunes which were such a struggle to learn that they always seem to stay playable, for me Alan Munde's 'Huckleberry Hornpipe' is such a tune. I spent so many hours learning that by ear that I suspect it will stay under my fingers forever!
When I can make time, these days I sometimes create a Tabledit tab of things I've just learnt. I then have something I can go back to if memory fails.

I find it depends on how often you played the song or tune, after you had first learnt it. Some that I only played or performed a couple of times tend to get long forgotten, where others I can remember note for note fifty year later, without any crib sheets or prior revision. Those that got forgotten though can usually be fairly quickly ‘relearnt’, as there is usually some of it still there in the recesses of the mind, and the muscle memory, if you can find the right prompts (such as earlier crib sheets, songbooks, or even some of the more accurate websites available these days).
I guess some people avoid this problem altogether by only sticking to a restricted repertoire of favourites, but then some of us always want to be adding in new stuff rather than standing still. Besides, some songs work better with certain audiences than others, so I think it helps to have as wide a repertoire as possible, but I am sure others might argue it’s better to get really good at a few songs, than to be adequate at hundreds of them.

I agree with others: it depends. I find that I can't remember breakdown type songs unless I play them very often. Vocal tunes are easy to remember and play (at least for me). I play by ear and maybe that's the reason I remember songs: I have a melody in my mind and that helps both my mind and my muscle memory kick in when I play and sing.

I think it helps if you do play by ear. If you memorize everything in tab then you're actually remembering someone else's version, not your own.

I've never had any interest in memorizing tunes. That's what sheet music is for. Sure, there are a few tunes that stick...but I've always been lousy at tune names.

Memory, interesting topic. As a folkie I have about 200 folk songs and 200 old time tunes I can play by memory, a little more with practice review. But hey, the data bank is getting overloaded and the aging (late 70s) infrastructure between my ears ain't what it used to be. I keep a list of tunes by key and regularly go over them. I can sometimes learn a tune by ear but I try to tab out everything so that "later" I can pick it up quickly as I learned it and be more ready for the next jam which of course is a review in itself,– play it or lose it! I take the name of a tune like "Jaybird" and make up song words to that tune for at least the first few measures with that title, "Jaybird jaybird yells and screams...." to be able to kick the tune off as a memory hook. I categorize songs into western, Irish, 1800s/ Civil War, modern that I can do 2 hour sets with each but I review the songs that I intend to do for events like a historical celebration or St. Paddy's Day.. Right now here in Ashland Oregon it is a rock and toll town and a bit of a folkie desert, I find myself just playing occasionally at the farmer's market, and I have a reenactment happening next month. Recently there was a rare song jam, about a dozen players of my vintage in the circle go around and every one of them had a i-pad in front of them except me. So an i-pad is an option although I think it is frowned upon at old time jams, and, then there is constant, systematic review as above that works good'nuff for me and maybe others, you? Best! banjered

Not here, but I read some guy's comment: "I keep my tunes in other people's heads."
Yep. I'd rather be a side man. I almost hate fiddle tunes. I can play them, but it puts me in the middle. I'd rather just take a break and pass it along.
I've always said, I don't know but few songs/tunes, but I know how to play my . . . . fiddle, banjer, guitar, bass, whatever you need.
So, no. Me got not much memory. Me got muscle memory, as they call it. Just let me hear it. I'll be there.

Some songs stick better in my brain, and others fall out of favor quickly. Sometimes there's no rhyme or reason. Brains are fickle sometimes.

The earlier in life you learn a song, the more likely you are to remember it. I am noticing that I have the same problem, not just with songs, but with names and a few other things that I should remember easily. That annoys me. I'm 61. What's it going to be like in another ten years?
I had something else I was going to say in this post, but I forgot what it was.

I play certain songs that are all memorized--some from way back--I hit them all straight thru (weekly}--and I piddle on just anything for a bit of my sessions--but I get thru them all-once a week--I still see flaws to fix or places to spice them up--as I progress in my thinking --if not my abilities--
My muscle memory does most of it, I like to slow them down and see the phrasing almost as a bystander--but ''too slow'' can actually be problematic--however the weak points are the first to show up ,,one has to pull more--to get the sustain needed for longer notes--that are caused by a slower tempo
my opinion--I ask no agreement

I agree with the comment about songs learned during YOUTH stick in my brain better than recently learned songs. A key factor (I think) is when I was young I practiced obsessively. Learning a song is just the beginning -- repeating it hundreds or thousands of times in jams or on stage nail it into my memory. I don't believe pne has really learned a song until it comes to you without "thinking" about it -- when it's more automatic, like muscle memory.

I've attempted to "learn" old time banjo since the 1990s. I've learned and held onto a few tunes by playing them obsessively, however I don't have any old time jams to play except once or twice a year at Banjo Camp or a festival. The last tune I tried HARD to learn was Adam Hurt's rendition of Lily of the Valley paired with Rocking in the Weary Land. Those two tunes really impressed me. I worked on them obsessively, and capped off my year long effort with a private session with Adam at Banjo Camp a couple of years ago where he SHOWED me how to play them (I can't read tab). I got so I could play it quite well. But I couldn't pick up the banjo right now and render either one of them. Because I don't play old time banjo in jams routinely. If I listened to Adam's CD, I could play them right away. But without that reminder - no.

I'm 72 and now notice how much harder it is to learn new bluegrass vocals. But I'm in a regional bluegrass band and we work on new songs every time we get together or perform. A couple of tunes I've struggled to learn are Hartford's I Wish We Had Our Time Again and Pete Rowan's Drumbeats on the Watchtower. What I've found helpful is running the songs mentally as I'm trying to fall asleep. This also helps me fall asleep. I've these songs nailed down pretty well now, playing them with the band.

I also compose banjo tunes, whether bluegrass or old time, in what I suspect is the Bill Monroe method -- the lines of the song come to me out of the air. If I like them I even name them. But can I remember them? Nope. I've lost a lot of nice tunes this way! I have one mandolin tune I put together decades ago and thankfully showed to a young mandolin player with a sharp memory. He has kept the tune alive -- he plays it beautifully, and I get to play it on guitar or banjo with him. We're teaching it to the band as a performance number -- as long as the young mando picker doesn't forget it!

So I guess my thoughts are: there's "learning" a tune, and REALLY LEARNING a tune through heavy repetition playing with other people.

I discovered a great forgotten Louvin Brothers gospel song yesterday that screams for bluegrass treatment. I'm playing it (alone) every day, and running the lines of lyrics at night as I fall asleep. I've shared the YouTube link with my band members and we're going to learn it together at next rehearsal, for a festival show this summer.

I wouldn’t worry about it. Maybe it’s more important to learn new skills and remember those than retaining tunes. When I was gigging a lot I remembered them, but no more.

I am 77. I agree that it is difficult to remember songs and tunes that I learn now. If I learn a new tune I can remember it for one or two weeks, after I played it last time. As long as I play it every day I keep it in memory.  when I have to play it later. However, I still remember tunes I learned 40 or 50 years ago, even without looking at tabs.

I once made a 20 year break of banjo, when getting married, raising children, building house, making carrier at work and traveling a lot. But after the 20 years break, when I started re-playing the banjo the muscle-memory (and perhaps music-memory) I could play at least 20 tunes I learned 20 years earlier, without looking at tabs (but listening music). 

I would say I have a pretty sharp memory in general, but there are fiddle tunes I had down pretty good over the summer on banjo that I would struggle to get through now. But I could probably get them back after a hour or so noodling around. Memory is a weird thing. I feel like I can recall faces, names, facts/figures pretty well over time, but episodic events from my own life are generally pretty fuzzy. I’m 36 now.

Say what ..... memory a weird thing?!?!?!   

I haven't checked into it, but I've read/heard that humans have the ability to recall any event they've ever experienced, IF the correct stimulus/conditions comes around. 

If I don’t play songs regularly, I will forget them. That’s one of the main reasons that I try to discipline myself to keep tablatures of any songs I learn or any arrangements I create. If I quit playing a song for a long period of time and no longer remember how to play it, I can get out the tablature and be back up to speed pretty quickly. It’s been my observation that re-learning a forgotten song from one of my old tablatures can be accomplished much more quickly than learning a new song from a new tablature. That suggests to me that the forgotten songs are still stored somewhere in my memory. The challenge is to figure out how to quickly retrieve them from memory.

Another observation I would offer is the association between memory and the modes of playing. I’ve found that, with respect to memory, there is a difference between playing and performing. If you learn a song, and you sit in your living room playing it for your own amusement, the memory of the song won’t be as strong as it would be if you play the song as part of a performance for an audience. I believe that the added pressure associated with a successful performance strengthens the memory. I think one part of your brain tells another part of your brain, “You better remember this bozo, because you're going to have to play it again”.

The problem with a lot of old tunes is that they are all the same only different.

my well being also comes into play --so one night i might flub things that are much better --- after some rest

Thanks for all the comments. OK, i am pretty relieved. Sounds like my memory might be somewhere in the broad “normal” range. I appreciate all the insights and thoughts about memory and playing too; very helpful.

I play a lot of fiddle tunes. Sometimes when I hear a fiddle tune I know my fingers seem to know what to do. Other times I recognize the tune but can't think of the title and my fingers don't know what to do. Once I get the name of the tune I can play it. As said previously memory is a weird thing.

If I've really memorized a song, it'll be there a year later, with maybe just a couple of notes or words I forget.

But ten years later without playing or listening? It doesn't matter what instrument, doesn't matter if I learned it by ear or by tab or by sheet music or even wrote the song myself and performed it many times, it's gone. It probably won't take long for me to relearn it, though.

Once you know a lot of songs, part of practicing is playing the ones you know often enough to not forget them.

This may wind up being one of the best most responded to post on here in a long time . The only problem is we won’t remember which post it was by tomorrow. Don’t worry brother . If something is wrong with you ? Something is wrong with a bunch of us .

I find that it depends upon the tune. If it’s in Scruggs style, it’s a lot easier to remember. I can never forget how to play Groundspeed or Shuckin the Corn no matter how long I go without playing them. If it’s a complicated fiddle tune or any tune in melodic style, if I don’t keep at it, I’ll forget it. I used to be able to play Nola, but after not playing it for about a year, it’s completely gone and it might take me a couple of days to get it back. It’s frustrating. I like playing fiddle tunes in melodic style and I keep a long list of those I have learned over the years. If I were more disciplined, I would try to practice them all on a regular basis, there some tunes on the list that I can’t even hum. But the good news is that there’s some degree of muscle memory so I’m not completely back to square one. I’d like to think that I’m only a couple of hours away from getting most of the tunes back.

Once I've learned a fiddle tune or any song I can always play it,but it changes in nuanced ways over time and multiple playings.It evolves with my mood and how my day is going.