This all makes me think about professional sports. In most, you have people playing from about ages 17/18 to people in their 40's. You'll see someone younger than that occasionally, like Freddy Adu started pro soccer in the MLS at 14, but most of the time not. Why is this? I think it's mostly because nobody wants them to get hurt, because they're not ready (even if they have the skill to play). If you figure they're good now, consider that they are also still growing and in 3, 4, 5, 6 years how better they'll be yet! There's really no need to advance to a higher level at that time. And being a pro athlete is a lot more work than being in a league for advanced youngsters in the sport, were they may shine and hone their skills, and still be able to do other things. Maybe be a kid, which is something people always say they miss when they get older. (is being a kid a bad thing? I don't think so, especially if you are one). An 18 year old and a 32 year old athlete may be on even footing when playing a pro sport but rarely will someone in their earlier teams be competitive with the best of the best.
They can be competitive with people older though, Feddy Adu was better than most human being period at soccer when he was 14, from 14 to 50. But if you're the "cream of the crop," why would you be interested in playing with people who make equal to you raw skill level, but much worse relatively? Good practice maybe, but not something they'd be interested in long term. IF you were to take 13 year old Roger Federer and stick him in the ATP tour, he'd be blown away and out of his range. Playing talented people his own age is what got him to where he is today.
That's just one analogy that came to mind, it's actually a bit specific looking back on it. Life is not like a line (like sport skill may be), where everyone starts at point A and strive towards point B. Being advanced in a field for your age doesn't put you on par with someone X amount of years older, because as we age we do not only mature but change as well. I don't like to think maturity is like a straight line, but as a wide open wide open field. We all follow different paths to get to the other side, some more scenic then others. It's a wonder why people sprint past the birds and flowers to get to the next part, only to wish they had spent more time looking around when they had the chance.