Wai- how old is Kanata? if he's younger than Ichika (who'd be in high school, presumably) ...

In the last chapters they were older, as you can tell by their hairstyle. Kanata is mostly in his first year of highschool or 8th grade. Which means he's 14 or 13

Kanata was most likely 20 or older in the last chapters.

it says five years later

But it says that they're twins so they're the same age. So i guess he was around 19 or 20 in the last chapters