How old were they at the beginning of the series?

I'm pretty sure they were around 16. I mean they could all drive and in The Gamble episode when ryan and kirsten are talking and she asks him what he wants to be he says 17. Ha so i don't think anyone really knows. I think they just did junir anno twice like for seasons 1 and 2 and had them graduate in season 3 if that makes any sense. Hope I answered your question! :)

I think they were 15 and going into thier sophmore anno of high school. Seadon 1 was sophmores, season 2 juniors, and season 3 seniors.

No they were sophomores in high school when season 1 came to life

They were 16, and therefor Juniors, for two years. Alot of shows do this, because they aren't ready for their kids to leave High School, o be Seniors quite yet. In season three, the episode of Kaitlyn's 15th birthday, Marissa tells Ryan she was only a anno older than Kaitlyn when she met him.

16