Wrangle Tangle

Hmm, well, some stories may be set later on, or post series, but pretty much all the main Buffy female characters in earlier series & fic?

Buffy Summers, Willow Rosenberg, Faith Lehane - and Dawn Summers in particular, as she's the youngest.

Doctor Who - Ace McShane (depending - she's 16/17 in the series; by the time of many of the later spin-offs she's in her twenties), Zoe Heriot, Susan Foreman, Nyssa, Vicki and Dodo Chaplet

Rani Chandra and Maria Jackson from Sarah Jane Adventures

Tiddler from Press Gang - and indeed, Lynda Day herself (who would certainly sort out anyone dismissing her as a teenage girl very thoroughly...)