Canonically, Youmu is like 30+ if memory serves me right, but I guess in Youmu-kun's canon she's like 14-15? Edit: Or well, I guess it would be possible for her to be as young as 12, since apparently a lot of people still count the years for this according to kazoe (long story short: it's an old age reckoning system by which babies are considered to be 1 by the time they're born, and a year is added to that on each New Year's day).
