I think there are a number of reasons for this. I don't have a lot of time to write it all out here, and I would like to get my thoughts more focused on the whole subject before I write too much but here are a few reasons that I think lead to a so-called loss of creativity.
Inactivity and stress leads to a whole host of issues, which I think loss of creativity is a side effect of. I suffered a severe loss of creativity during my early to late 20s. I attribute this to high stress, too much work, and a loss of focus. Also depression didn't help at all.
Inactivity and stress makes you feel like crap. So does depression. I would go through bursts of creativity even at my low points, but I could never finish anything I started. Depression would always make me fail. Learning to live with depression, getting rid of my stress and focusing in hard on the things I want to make has boosted me to (what at least feels like) a whole new creative level.
I think people lose their creative focus. They forget the reason for creating in the first place and so they see no reason to remain creative perhaps, or they can't see whatever it may be they enjoy creating panning out into a profitable business, so they stop. They allow social norms to smash their creativity at various critical stages in their life perhaps. Some parents are pretty down on creativity as well. Society as a whole celebrates people who think differently, but in day to day life the people you encounter openly show their disapproval (if you speak your mind).
If you don't practice your creativity, study techniques, test yourself, fail at whatever you're doing and learn from your mistakes (just like literally everything else) you will end up "losing" your creativity... though I like to think that some people who were "creative" never really found it in the first place. They perhaps started down the right path, and then abandoned it, like a child who gives up on reading. During my youth (I can remember as young as 5ish but I have one story saved which i wrote at age "3 1/2") I created things nearly every day. I have piles of papers saved from nearly every stage of my life, be they sketches, doodles, or something more. Most of it is unfinished and pretty much rubbish. I like to look at that pile of garbage and compare it to what I have now (just like I do with the stuff I made a few years or months ago).
I think that kids are just less inhibited, are able to make weird connections because they don't know how the world works (this can be simulated though, so don't despair), have less stress, less obligations, have more time, are healthier and more active like you said.