If I'm at any sort of social function at which families are involved, you can always find me with the kids, playing tag or hide-and-go-seek or whatever else they may be up to. Again, it's not an attempt to prove or recapture anything; rather, it's just a way to have fun. Adults have an annoying tendency to just stand around talking about the same old stuff, not using the gift of their bodies for anything other than standing up straight (or sitting on a chair) and holding a cup of something and talking. Kids, on the other hand, use their bodies to have fun, and in doing so they don't diminish the amount of energy available to them, but they actually make that energy grow so that they can have more fun later doing something else.
You see, kids haven't forgotten yet what it truly means to have fun. They haven't forgotten the thrill of movement and of play, and they haven't forgotten what it means to accept anyone as a playmate who's willing to play. As they grow and learn from their adult role models, they learn to discriminate between potential play partners, and they learn the "joy" of accomplishment versus the "waste" of time spent playing.
I often think that we have it all wrong. Somehow, we believe that it's our job as adults to teach kids how to grow into being just like we are. That belief is faulty, though. Actually, our higher calling is to learn from the kids--to learn all the things that we've forgotten about seeing and feeling and loving all the joy and wonder and fun that's here in this world for us. I like it when my wife laughs at me. Then I know that I'm learning my lessons at least fairly well.
When they tell you to grow up, they mean stop growing.