Lelo, of Witherwilds, is my youngest character. She's 11, going on 12. My oldest is High Priestess Valsidire, from The Traveler's Daughter, and she's a couple hundred years old. She is descended from gods and knows a secret to staying young and, just as important, alive.
In terms of creation (again, I'm confining my answers to my novels), Bria (The Traveler's Daughter) is the oldest. The germ of her story came to me in 1999, and she made it onto paper in 2001.*
Funnily, the entire cast of Witherwilds is the youngest. The youngest of the young is probably Qeoe, but actually, the five POVS in book one occurred to me almost simultaneously, on February 7th, 2008, to be precise. It has always been a story with many players, not one character's story where the others clamored to have their say.
* I've been writing for nine years! Where does the time go??? I've seen in many places, and most recently in rosefox 's Publishers Weekly article for Genreville "Advice for Young Writers and Editors," that it can take ten years from the date you start to write seriously to when you write something worth reading.