Most of the things that people stop doing because they're "too old" for them, are the things that give you happiness. Main reason why so many adults are miserable sods with no joy in their lives.

My potted theory.

I like cute stuff, which isn't something that I'm doing as such, but is the same kind of thing. I have a very cute bag that I use when I'm wearing my hot pink winter coat.

Both the bag and the pink coat are a "younger" thing to do, I'm sure, but I get compliments on both. That tells me that other people like them, and, that it's unusual enough to be comment-worthy. As so many people wear boring black winter coats and have boring black bags...

I'm utterly convinced my mum would make some comment about my bag being childish if she could think of a way of saying it without offending me. The fact that she's avoided refering to it at all says it all to me xD My sister, who's 31, thinks it's cute...

If something you enjoyed when you were younger still gives you pleasure now, keep doing it! Because the nostalgic element that is bound into that enjoyment makes it so much more of a good thing. Most people are at their happiest as a kid (see my initial statement) so doing something as a "grown up" that gave you pleasure as a kid helps you to relive a part of the unfettered joy of being a kid. Wonderful for your stress levels, and probably helps you to remember some of the good aspects of being human, instead of focusing on the bad, which is what a lot of adults do.

People forget the simple joy they felt as a kid, if there are ways out there that let you recapture that feeling, why deny yourself this because someone else thinks it's immature behaviour? Screw them, it's your life not theirs.