User blog comment:Gideoncrawle/New Year's special - LTDI update and preview/@comment-1590625-20110225024810/@comment-1874924-20110225033214

I don't know what the rule is in Canada, but in most (if not all) of the U.S. the cutoff is either 31 July or 1 August. Thus, someone with a June or July birthday would be 15 years old throughout their entire sophomore year, whereas someone with an August birthday would be 16 when their sophomore year starts. Sadie is in the former camp, because this scene implies that her birthday comes before the merge. If anyone on the show had an August birthday, then, yes, that person would be turning 17. The canon TDWT suggests that this is Cody's situation.