Addison Dixon I'd say 13 and up. My cousin--who read it when she was 13--loved it and read the entire series.
Person 11-infinity!!! Read it in class. They are some YA parts. Some mature scenes. There are kissing and intimate scenes. The movie shows these scenes too. So...yah
Jeremy Pennington I read this book when I was 9, I don't recommend doing so though. There was a lot I didn't quite understand. So I reread it when I was 14
Lindsay I'm in my mid thirties, got through the first 12 chapters, but had to put it down. The writing feels quite YA to the point of spelling everything out. It was too simplistic for me (I had just finished A Handmaid's Tale and it was a large departure).
Trekkie I would sat 15/16+ there was a pretty descriptive kissing scene and even more in the rest of the series
Emilee Schneider I read it when I was 10. It was really good and I understood most of it but there was some mature stuff
Olivia I read this when I was 11, but I was pretty mature. I think you should be 12 when you read this. This is an amazing book!
Mrs D Whitehead I don't think it matters how old you are - I'm in my fifties and am a hard audience to please and I enjoyed it. Don't pigeon-hole it!