They're not entirely for little kids. I assumed they were crap, but finally decided to read them, and enjoyed them quite a bit. I'm 22. Off the top of my head, I know my 20-year old brother has read them; my 23-year old friend and his 20-year old wife; and my friend's 20-year old fiance. They're actually reasonably popular among the college-age crowd.
The movies were pretty godawful though, and LOTR is better overall.