I've noticed the same thing in reviewing the ages on applications to my VA. You have a group between 13 and 18 that want to tear up the skies and cant afford the lessons to fly for real, a large gap from 19 to 25/26, and then players evenly spaced from about 27 on up.
I think that as the youngsters discover girls and start to drive that flight sim, for that matter almost all games go by the wayside as they start life.
Once you are in your late 20's and above you get secure in your future, you have a wife of significant other and maybe a kid or two. You cant go run around anymore, you have very little money for funstuff, but you have a PC and a joystick. Well you fly the world, talk via Teamspeak to many people and the wife knows where you are at at all times.
Flight sim becomes more social as you get into it, not a bad thing considering that unless you have wads of money or join the military this is the only way for folks to experiance the world.
Regarding Combat sims, where else can a grown, "mature", individual act like a kid and not have to worry about looking foolish? And it's cheaper than Air Combat USA and like operations.