The bounce back game is real as a kid.

Maybe it's all about mass. The force on my knees landing at 200lbs is way greater than at 50lbs.

But maybe I'm just trying not to feel old.

Mass, height, and the mechanics of momentum (as when, say, tripping) all make a difference. Some of the terms involved have exponents applied. Adults definitely feel a lot more force from mistakes made while moving their bodies than kids do.