Aging -vs- Our inner child

Getting older is such a weird thing. I mean, from like 13 years old on, our minds are already pretty much formed in terms of our ideals, goals, and expectations from life. Granted, those things gradually change as we grow further, but for the most part, we still have a lot of the same questions throughout our entire life as we did when we were young.
Meaning years are aging our bodies, but not necessarily all aspects of our minds. It's kinda sad how we are expected to (once adulthood is upon us) be immediately and completely mature. No time to "play around", no time to enjoy life. (I'm being a tad bit facetious here). I know adults still have fun, but not the same fun.
I think a lot of us "adults" are still youthful at heart. So why is our inner child being stifled? Perhaps I feel this way because I am in a gray area as far as age. I am an adult, yes- but I am still young in the eyes of the world. I find daily conflict within balancing the two. But why do I have to balance at all? Why can't I enjoy life with the passion and zest of a child? Is it not P.C. or something?
Another thing that I was curious about, is when do we start losing that sense of youth and again, why? Mind you, I am not talking physical youth, besides diet and exercise, you can't really prevent bodily aging. Psychologically speaking though, why are we sooooo serious as adults.
I feel that people who lead responsible lives, and take care of their business should be able to lead their lives as they please. As long as they don't harm or inhibit others in the process, of course. I feel that judgement is wrong. Granted, we all judge on some level- I don't think we can help it as a society- but for the most part, we should let others lead their own lives in peace. With that said... go to the park, watch a cartoon, buy some rollerskates... either way- I think we should all go and spoil our inner childs!
:)