I left age ambiguous because due to in the game, one character stays behind as a third year until she is able to defeat her long lost friend in a duel. She is 25 I believe. So based on your characters bio, you could have them at any age, although I would prefer them to be older than 13.
But generally:
1st year = 15-16
2nd Year = 16-17
3rd Year = 17-18