I do this because they're inexperienced when they start the game, and they develop. Much how my roleplaying goes.
So, I voted 17 - 20 because even when I play Dunmer (who mature older, and live longer) they tend to be age 20 - 25 which is roughly 17 - 20 ish in human years (maturity terms) I believe. I'm not entirely sure though.
What about you?