Older but not wiser
As we grow older, we become more bitter and depressed. Some would argue that this has to do with being confronted with the raw realities of adult life. However, many children are still able to play and have fun under brutal circumstances.
I think the explanation is simpler. As we grow older, the number of new experiences diminishes, and we recognize a lot of "new" things as revamped old stuff. As children, we're still lost in wonder and marveling at everything we see. Staying young, then, is all about keeping an open mind and actively seeking out truly new experiences.