Yeah, basically what Masamune said. I found that as I get older, I became more self concious about creativity because I was comparing my ideas with others that already existed ( in film, tv, games, art, music etc) so I became more critcal and scrapped ideas at a very early stage if I thought they were too derivative. Children generally dont have that awareness, or if they do it doesnt affect them, theyre just happy to do their own thing.
I suppose the trick is to be aware of self censorship/editing, but not allow it to control your creativity. Like Masamune said dont let it become 'work' because that is the enemy of creativity. The childlike mindset is doing something because you enjoy it, not as a means to an end.
For example, years ago I had to get a portfolio of artwork together to get on a course. I didn't really have very much stuff that I had done (altough I wish I had kept it) Anyway I spend 2 nights just churning out sketches and paintings. I really didnt enjoy it and although the work looked OK (they let me on the course!) It didnt reflect who I was 'as an artist' (ugh I really hate that phrase)
So probably theres nothing wrong wtih your imagination, its just that the adult world has piled a load of crap on top of it so youv'e got to dig it out and dust it off. And dont force yourself to 'create' either, by all means keep drawing stuff to hone your skills, but dont feel like you should be creating masterworks all the time.
Hope my jibbering has helped a bit.