Board Thread:Legitimate Canon Questions/@comment-46.5.2.35-20150603194815/@comment-4091815-20150603211644

Scott was 16 in Season 2 when Victoria Argent tried to kill him - he should have turned 17 by the episode you mention but I don't remember any exact line where he says he is older than that.

Allison wasn't a full year older than the others, she was mearly a grade behind where she should have been in school.

Lydia is about to turn 18 at the end of Season 4.

While we have no firm age for Stiles in the canon, he is believed to be at least 16 in Season 1 since he can drive. He says in Season 3 (Chaos Rising) that he's not yet turned 17.

Keep in mind that only one year passed within the Teen Wolf universe between the start of Season 1 and the end of Season 3 so their ages aren't that much different.