And gravity. I started wrestling (and judo) as a kid, when I weighed maybe 70 pounds soaking wet. Gravity didn't mean much back then, takedowns and falling (including off of trees and monkey-bars) were painless; you hit the ground and were back on your feet like you were made of Indian rubber. Didn't even matter how you fell, you were so light it didn't matter; eventually you got good at it. Plus your bones were half cartilage and so most of the time bent rather than broke, and even when they broke healed so fast it didn't matter. Plus all the cute girls would come around to sign your cast for the week or so you had it on.
As an adult I'm used to being taken down/thrown, but even with much better breakfall than I had as a kid starting out, I feel it much more - doubling your mass, even if most of it is fit weight, does that, as does being thrown from a higher height by a heavier opponent. I can't imagine starting as an older (ie 30+) adult; the falls alone would be reason to find a different activity. Like ground oriented BJJ.
BTW, this applies to a number of sports. Its why adults almost never learn to ski as well as people who started as kids. Falling on skis as a child is actually fun, you're so close to the ground and so light there's no hesitation to try things. Falling as an adult is not so good. And there's no bragging rights showing up at work with your arm in a cast.
Because no one wants old men walking into the high school wrestling gym to roll around with the younger boys.
