Board Thread:Administrative Requests/@comment-24564327-20150514143307/@comment-25555436-20150514144634

Exactly, those should an obvious enough reason for why she is 15. She's not younger than 15 (physically, at least).

1-The age gap is of 2 years. As Weiss, Blake and Yang are 17, and Ruby's 2 years younger than them, I assume you know basic math.

2-She was bumped ahead a few years. Those few years=2 years, just like she said in the very first episode. ("I only have 2 more years left at Signal...).

3-People routinely mention her age in-show? Really?