Because the explanation, of course, is very simple. Kids have a lot of free time, adults not so much, and practice time is the biggest factor in being good at almost any competitive activity.
Yeah, it's a whole lot harder to put 20+ hours a week into a game for several weeks on end, which is what it usually takes to get pretty-good-for-a-non-pro at a multiplayer FPS, when you're middle-aged with a job, a spouse, and (especially) kids. And still trying to have any kind of IRL social life whatsoever. Only really do-able if you're willing to give up most other interests for that period, and probably sacrifice some sleep, too.
