Are we all just tainted from years of adulthood?

When Christopher Robin left the Hundred Acre Wood to go to boarding school it was basically the end of his childhood. We let children have unlimited imagination and support their creativity when they are young but at a certain point, parents are required to get the child ready for a more definitive world where certain things are expected out of you and playing around with dolls or just being goofy isn't an option. But when I think to my happiest times, it wasn't learning mathematics or getting more responsibility or a job, but those times that I was truly free to think and be what I wanted to be. I look back at pictures of me as a child and I see an actual full blown smile and not just one I put on for the picture subconsciously whether I'm happy or not. When did we lose that feeling and is there a way to get it back?