Of course, for some sports, there's only so long your body can stand up to it. Gymnastics for instance is very demanding - and when you actually go through puberty (most are delayed) your body often goes through changes such that it is difficult to do the moves you could do before. Becoming too heavily 'endowed' can really mess with your balance. Didn't watch too much gymnastics, but wasn't there a Russian (think I saw the vault) who was attending her 5th Olympics? So she was like 30-something? that's ancient for a gymnast.
I'd think the stress in general of achieving so much at a young age would get to a lot of people. They might be happy to lead a mediocre life once they're adults.
"Up plus down equals flat" Pumpkin, 3 yrs, 10 mo, July '07