I think the younger one is 16 and older one is 18. It'd make sens then, because the difference beetwen 12yo and 14yo can be quite big (since the younger one was still in elementary).
the older brother can't be 18 and still at school, well I don't know, maybe the younger is 15 while the other is 17 ╮(￣▽￣)╭
You know, I just always assume that in 3 grade of high school people are 18yo or that they gonna have 18 birthday in it :p They same goes with the rest of the ages ╮(￣▽￣)╭
somebody's got the age gape between the brothers? because the older one is still at school and the young too but in the flashback they looked like years apart
PS: my favourite was the older brother's story <3