this is not to contradict the last ask, this is just information... google says that at the beginning introduction of Koito, he would be 21-22, and Tsukishima is 34. that would be a 12-13 year age gap.
my personal tentative opinion, is that this is potentially not that bad. they are both adults, and this isn't even banking on Koito having just passed the age of consent... he's fully not a teenager by a year or two. but as with any age gap between adults, it depends on the dynamic in the relationship, and how it's handled. it could still be creepy if it was written to be, but I don't think that's what anyone is in this for, from what I've seen. I think a lot of people rest their enjoyment of the ship on the fact that Tsukishima is so dry and practical, and they have a dynamic of Tsukishima looking out for Koito's wellbeing. meanwhile Koito's arc is about him gaining a specifically more adult level of maturity, before which point I definitely don't think Tsukishima would be interested in that way. early in the relationship, Tsukishima definitely finds him just plain annoying, but I think most soldiers do, because Koito is basically arriving to the situation late, after they were all already traumatized by the war. there's something irritating about his spotless naivety, which is interesting because that's kind of a theme, especially with Ogata and his whole deal. but what separates others from Ogata is that they temper their bitterness towards the concept of "the innocent" and they don't just write people off for that.
ultimately, the soldiers each heal when their more harmless side, which they walled off during the war, is able to be part of their persona again. Sugimoto has the makings of a very sweet, completely harmless human being inside of him. it didn't do him any good during the war, so that piece of him got compartmentalized and put away... but it's still there, and Asirpa brings it out of him. obviously there's nothing romantic between those two, and there doesn't have to be anything romantic between Koito and Tsukishima either for that to work. but similarly, Tanigaki has Chikapasi and Inkarmat, so it can be a romantic/familial tie that re-humanizes them as well. this is, I think, what people see in the ship.
[[Didn't think it would turn into a whole thematic analysis. Love that re-humanization angle you took, anon!]]