I was under the impression that lydia is 16/was 15 until season 2. I was pretty sure they were all sophomores, which would be reasonable because a good deal of kids are, like... 14 in 8th grade, 14/15 in 9th grade, and 15/16 in 10th. (Though really this is weird I have never heard of a high school that has a sophmore captain/co captains of a varsity sport, though in the first episode, Lydia does say they've had a "winning lacrosse team" for three years while praising Jackson, which makes me wonder about a lot of timeline and continuity stuff in this show...)

Also why would Allison have had her little freak out moment about turning 17 if all the other kids in her year were turning 17 too? I don't think she would have. And if Stiles is 17, too, it leads me to believe that, yeah, they were both held back.

I really want to know where the birthdays on the wiki came from, because a lot of them seem off year-wise.