This answer contains spoilers… (view spoiler)[I read it at 13 (circa 1976) and really enjoyed it. I disagree with Tom. It's *not* dumb and as dysfunctional utopia/future Earth books go, this is a really fun read (with a hopeful ending). However, there are some sexual situations to which you may or may not object -- if you're considering letting a child read this. Lastly, as is the case 97% of the time, the book was superior and utterly different to the film of the same name. (hide spoiler)]
Dana Emerick Yocum Due to some adult issues, I would recommend high school age. My daughter has yet to read it
SamSpayedPI The book includes sexual situations but not explicit descriptions. I read it at age 10 or 11; I remember having to look up the word "orgasm" in the dictionary.
This answer contains spoilers… (view spoiler)[While reading level would probably be 7th grade, I'd say late high school because of the sexual situations. There's a "pleasure center" you can go to in a glass building where you can sleep with a prostitute while seeing other people do the same. In another scene, a main character goes out with a group whose intention is find and record people having sex. In another scene, a band of gypsies kidnap and force the main characters into sex with each member of their tribe. The descriptions are not gratuitous, so they'd probably go over younger kids heads. But I wouldn't recommend this book to anyone younger than a junior or senior in high school for that reason. (hide spoiler)]
William For the language used, "high-school" level. However, there are some sexual themes and situations. There's also a considerable amount of graphic violence, but hey, we Americans are OK with violence...just no sex.