Responses
I think Yuki is 20 and it's mentioned that Natsu is either 10 years older or more than that.
Hikari was became 2 years old at vol. 1, but I'm sure he grew up after that, maybe 3 or 4? :P
How old are characters? Oto? Or twins? Masaki? Hiromu? Matsu? And Yuu-kun? It was stated anywhere?