[Yapyap]

There's a lot of "children's" series where the fandom is full of 15-18-year-olds (and older) - Percy Jackson, Harry Potter obviously back in the day, Skulduggery Pleasant, etc. I don't think the lines between "childish" and "not childish" (or MG/YA) are that strict these days.

For other kids to make fun of someone for reading books that are "too young", I'd imagine the reading material would have to be something very young / aimed at 7-9-year-olds, not so much Tamora Pierce or Diane Duane (also, I'd doubt kids who haven't read those authors themselves would know to make fun about them, and kids who have, probably wouldn't make fun because they really aren't actually that "young"!).

Anyway, I'd agree with most of the suggestions made. I've only read one series by Tamora Pierce (The Immortals, which was okay although not outstanding) but I wouldn't consider her stuff "too young". Diane Duane ... I enjoyed the books (I haven't read the updated versions) when I read them a few years ago, well past the age of 15, but I think that will really depend on a specific 15-year-old: some will like them, others won't.

Sexuality in books is a tricky matter when picking things suitable for a 15-year-old. On the one hand, this is exactly the age where kids are figuring themselves out (and gender and sexual diversity is, as I've learned from being in fandoms with 15-16-year-olds, Very Important and they crave representation and diversity probably more than any other age group) so I'd never, ever go "these books handle diverse situations/characters and are unsuitable for teens" (you never know, the teens in question may be privately struggling over figuring out what they are - the parents are often the last to know!).

On the other hand, there are plenty of 15-year-olds who really don't want romance-focused books or books with more than a passing reference to sex or sexuality, and that should certainly be kept in mind, too.

In any case, a mature 15-year-old is basically what I'd consider ready for anything, so I'd just say whatever catches her fancy from MG books to YA to adult books, but a lot of adult fantasy / scifi books these days do have some more explicit stuff in them. I just skim those parts, but, well. This is probably why reccing MG/YA books seems like a safer bet.

For some specific recs, the fantasy / scifi books aimed at younger readers which I've enjoyed in recent years, being an adult who happily reads things like that (and finds that a lot of that doesn't work for older readers as well, while some does), include Skulduggery Pleasant (Derek Landy), the Bartimaeus sequence (Jonathan Stroud), Percy Jackson (Rick Riordan), The Hunger Games (Suzanne Collins), Unwind (Neal Shusterman), Never Let Me Go (Kazuo Ishiguro), almost anything by Scott Westerfeld (Uglies, Leviathan), the Chaos Walking trilogy (Patrick Ness), The Lunar Chronicles (Marissa Meyer), Earth Girl (Janet Edwards), Katya's World and Katya's War (Jonathan Howard), Half a King (Joe Abercrombie), and .. well, lots more, but those are all I've appreciated for actual deeper themes and that also aren't too heavy on romance (The Hunger Games and Lunar Chronicles are probably the romance-heaviest on the list but it's not the focus in neither series).