it hasn't been mentioned, but haijin is a minimum of 5 years older according to the storyline so far, and jk is finishing high-school soon so she's probably 17.
Although haijin could be in early 20s, I'm leaning more towards him being in his mid or late 20s.
I'm not sure if you read it so I'm going to put spoiler since I mention bit from some of the chapters.
MINOR SPOILERs^^
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I don't think its been clarified yet but I'm pretty sure the first time they met was when he was a highschooler and she was in elementary, remember when she painted a forest and talked about a memory?
That forest memory is actually quite the plot point, though since your just asking about age... I won't say anymore.
I'd say haijins atleast 5 years older, I'd take a guess of about 8(?) years older since he is also older than mikotos brothers.
Mikoto is also in her last year of high-school? I believe she is 17 as of current.
There's actually quite a bit of spoilers you can find digging around the comments.
Probably 8-10 age gap, not sure tho.
Thanks guys :)))
How much is their age gap?