I think there's nothing wrong with reading YA books as an adult. It often seems like there's more originality and variety than in books aimed at teenagers than those aimed at adults. They're often (with some sparkly exceptions

) well-plotted, with memorable characters and engaging themes, and that's pretty much what I want in a book. Plus, I think that YA fiction is considerably better now than it was when I was a teenager.
I think the only thing to remember when reading YA fiction is that the main characters are teenagers, so there's no point complaining that they're acting like teenagers.
