>It's not really THAT noticable after 18, though, so it is not a thing to consider after that.

I don't think you can define such a hard-and-fast age at which you stop mentally maturing, though. I know that I was a much different person at 20 than I was at 18, and that I'm a much different person now than I was at 20. Maybe in another two or four years, I'll be a different person again. Maybe it never ends.