I'm shit for figuring out what's too "old" for someone, but fairies, elves, urban fantasy? I am all over the Toby Daye series by Seanan McGuire. They are absolutely wonderful. The first one is _Rosemary and Rue_. October Daye is a changeling, half human, half fae, private investigator. There is no explicit sex. People do, eventually, have sex in like book 4, I think. There is violence, but it is not prurient. Child abduction is a theme throughout. There are, um six or seven books out of, I believe, a total of 10 planned.

For the 12-year-old -- maybe 100 Cupboards / Dandelion Fire / the Chestnut King?

Also, if she hasn't read it, the Spiderwick Chronicles.

For the 12-yr-old, the Bordertown series, in which Faerie intersects with our world somewhere near Los Angeles. There is sex, but it's responsible and not graphic. It's a shared world; there are at least 2 novels and 3 anthologies.

For the 10-yr-old: Michael Chabon's Summerland (involves baseball, multiple worlds, and Norse mythology, among other things). And 3 nonfiction recs, depending on his interests: T. Rex and the Crater of Doom by Walter Alvarez (paleontology); Cuckoo's Egg by Clifford Stoll (computer espionage); Ravens in Winter by Bernd Heinrich (biology).

I'll pass on the 14-yr-old because I don't know from manga.

Nausicaä of the Valley of the Wind by Hayao Miyazaki for the 13-year-old if she hasn't read it already. Environmentalist fantasy with kickass female characters and also giant spiders.

For the 12-year-old, Un Lun Dun by China Miéville: portal fantasy set in London with wordplay and excellent trope subversion. Series-wise, the Weetzie Bat books by Francesca Lia Block are really good—magical realism with awesome queer characters.

For the 10-year-old, maybe Little Brother by Cory Doctorow (it's YA, but takes its young audience quite seriously). Railsea, also by China Miéville, is quite cool; it's written at a pretty mature level but has a young protagonist and is substantially less violent than his other stuff. Under My Roof by nihilistic_kid is tons of fun.

Off the top of my little head, two more excellent fantasy authors (though no series yet): Nnedi Okorafor and Karen Healey.

I have just purchased some Pratchett (The Color of Magic and Hogsfather) for my 12yo nephew, and some Jim Hines (the first two princess books, The Stepsister Scheme and The Mermaid's Madness-- short on real-world elements but long on fairies and kickass women) for my 15yo niece.