I'm 27 and started metal detecting when I was 13--and I'll still be detectin' when I'm 80 if I can! I have also found that many detectorists are older. My young friends think it is an absurd hobby, and many of them think it is something people should save for their retirement years. Now, it is true that I don't have as much time for metal detecting as I'd like, but if I had my way I'd be out every day with my Fisher 1266-X. Metal detecting seems like a hobby that you never give up on (unless you had the misfortune of your only experiences with it coming from a $50 Radio Shack detector!). Those that got into the hobby in the 60's and 70's have really got some stories to tell, and most of them will never give up on it (they've just found so much cool stuff). I like to call that time period the "golden age of metal detecting" when every site--even the big ones--was a virgin site. The only thing that has changed is that it takes far more research and a little luck for us now. For example, I had to find my first Civil War buckle in a church yard (battlefield and campsite pickings were getting quite slim). I met an old timer once that said he hunted Civil War sites thirty years ago. He said he only dug the large targets (buckles, cannon balls etc.) and left the bullets because there were thousands of bullets. While there will be places for us to hunt for years into the future, we now have to contend with picked over, "worked out," and "hunted to death" sites. By the way, I see these as a challenge. I have pulled some fine coins and relics from previously hunted sites. It's just more difficult.
An excellent question to ask, though, because I have always been curious about the average age of detectorists. Good luck folks!