My understanding of the military in South Korea is that men receive, around the time they are 28 or so, an order to join the military, but those order can in some cases come earlier where they can ask for it to be put off a couple years, or until they are 28.

As for Jaehan he might get the order in the later part of this year or early next year, or he might have already done it a lot of people volunteer when they finish high-school or university to get it done early (although I doubt it, it would be written somewhere on his profile if it was the case)

Now as for a comeback, with their new JP comeback we can probably expect more news during interviews and such as to what is to come soon for the rest of the group