They're learning to ski, surf or skate, or something fun.
Here's how I see it. I'm 17, when I'm like 30 they may have some crazy artificial body parts. More high tech knees and hips and such. Maybe even artificial ligaments, if they don't already have that now. I could fall on a rail, get up and notice the rail is dented cause my hip is steel/carbon fiber. The body won't be able to fail, only the mind, and unless I get my brains knocked out (knock on wood) I'm skiing fast and hard forever. "Yeeehhhh, back in the day I remember when Keeeveen Roollannnnd did three dubs in one pipe run, he got like a 94." "Where his other tricks all switch 1260s, that's a high score for only three dubs," says some little kid.
I don't mean turn into the terminator though, more metal than bone would be bad. This brings up a whole new topic; when will athletes undergo these procedures voluntarily? When they realize they've reached the limits of the human body, that it can't survive a triple backflip off a 300ft cliff with just bone. Who will be the first to say "these bones break too easily, replace the hip please." +k to whoever invents the anti shinbang shin implant. Will any sport even get so far as to need it?
Rant? Crazy unrelated idea? Yes. If you read all that and found it interesting, cool. If not, whatever.