Actually, having a childish decor that your friends might tease you for is pretty easy - "yeah, they did it like this when I was 6, and they keep promising to change it..." After all, it's not like every kid matures at the same age, and I doubt I would have complained about a comic-themed bedroom at 9 or 19... And know that I say that, I haven't redecorated my bedroom in years
Kat, I kind of disagree with you about the generic decorations. Personalizing a kid's room is nice, and it seems to me that most rooms should be cleaned out, thoroughly cleaned, and repainted every four to six years. On the other hand, that probably depends more on your desire to do redecorating, as much or more than your ability to afford it (and your time), so I'd guess it's something that's individualized per family.
Except the paddle part, of course.
Full Disclosure: I did used to let my kids do some personalized decorating like this, but we've gotten away from it, as more and more of the kids have to share, and now we do go more with what Kat suggested.