I love paranormal series and whether they are shelved in the adult or teen sections, matters not to me. Some of the adult series that I think teens might like are:
Hollows
Dead Witch Walking by Kim Harrison
Rachel Morgan, a witch and bounty hunter, works in "the Hollows" of Cincinnati where vampires rule the darkness. There are 10 Hollows novels to read!
Greywalker Novels
Greywalker by Kat Richardson
PI Harper Blaine is left for dead by a savage assault. When she awakes in the hospital, she discovers that she can move between the human world and one of vampires and ghosts, magic and witches, necromancers and sinister artifacts. There are six Greywalker novels.
Kitty Norville Series
Kitty and the Midnight Hour by Carrie Vaughn
When Kitty Norville, a DJ for a Denver radio station and a secret werewolf, accidentally launches a late-night advice show for the supernaturally disadvantaged, she becomes the target of a werewolf hunter and several homicidal undead. Nine Kitty books are out.
Mercy Thompson Novels
Moon Called by Patricia Briggs
Mercy Thompson's life is not exactly normal. Her next-door neighbor is a werewolf. Her former boss is a gremlin. And she's fixing a VW bus for a vampire. But then, Mercy isn't exactly normal herself. There are six Mercy Thompson novels.