Hanna If you've read a lot of books with a romance in it (I mean like Twilight-romance) you could definetely read it. I've read it and I am 11 and I love it. But everbody is different and I don't know what you like to read about. If you don't like the sex scenes in sme books I don't recommend this book to you. But that's just my opinion.
Katie I would say (In my opinion) 13+ if you are in year 7(what we call 6th grade here in the UK.) I would not read it I feel like I could of read it later since I read most of the books when I was 12 which was ok but I felt a bit uncomfortable reading the hot scenes (there is a tiny bit of sex but it does not put it in to much detail.) I would say wait till your about 13/14 if your fine with the stuff that I said GIVE IT A GO! its a really fun series and most of the time (maybe not in the 3rd book!) the romance takes a back seat! it has really cool sci-fi and cool aliens you will love it!
Trisha There are some pretty hot and steamy scenes as well as objectionable language. In my opinion, and as the parent of two teen-aged girls, I would definitely not let an 11 year old read this series. It's more of a 15+.