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 year 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 year 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, or be Seniors quite yet. In season three, the episode of Kaitlyn's 15th birthday, Marissa tells Ryan she was only a year older than Kaitlyn when she met him.

16