Board Thread:Speculation House/@comment-24.247.91.107-20140511154823/@comment-4010415-20140825103832

Soulless Doll wrote: if their moms and dad were on the same team then I think that Cinder could have been the fourth person on that team. Except Cinder is in her early 20s, way too young to be on the same team as Ruby's mother, given that Ruby is 15.

Monty said that Cinder is only a little older than the students. Students graduate Beacon at 21 years old, and he said that Cinder isn't much older than them and is "young enough" to pose as a student. She could be posing as a senior student, still looking close to 21 while not being too far off from being 21.

Source for graduates being 21: Students normally start attending Beacon at age 17 (Blake, Yang, Weiss, etc. except for Ruby are all 17), and they attend the school for 4 years (says Ozpin). 17 + 4 = 21, so students should be 21 by graduation day.