In my possibly outdated, totally anecdotal experience, teens do like to read about twentysomethings, but they know how to find those books in the various adult sections—SF, romance, horror, general fiction, whatever. When they go to the YA section, they're specifically expecting coming-of-age books about people close to their age, and that's what they'll find.

Personally, I read lots of adult books from about age 12 on, but I still enjoyed YA books. I didn't see them as simpler or dumbed-down or anything like that, just more sharply focused on my age group. Also, I think I felt "safer" with YA books, like the author wasn't going to spring a graphic rape description or a totally nihilistic ending on me. That rule did not always hold, though, and still doesn't!

In terms of sex, I think it depends partially on your genre. In contemporary, especially "gritty" contemp, you can get more graphic. In a genre like mystery or fantasy, probably less so, but it depends on the story. I've heard that a certain very, very popular fantasy/romance series has an NA heat level.