Board Thread:Questions and Answers/@comment-34129530-20140923205051/@comment-25190958-20141030005713

The way I see it, they were fifteen in season one, and in season two it's actually a year later. Because Kirksman was working on the case of the missing scientists 'last year' he says, in a season two episode.

So I'm assuming that in Season 2 they're sixteen, and sometime later, like near the end of Season 3 they'll be seventeen.