Yeah it's a pretty basic age gap. In 2 - 3 years Hatano will become a college student and mc will graduate college and get a job.
Yep yep, this is something I commented about ages ago and did the same maths - 2 years by the least, 5 the most (but shorter is much more likely). That kind of age gap hardly means anything when you're past 20 (at least where I live), and since the manga itself has brought up the fact that Hatano is currently still in high school, I doubt anything serious will happen until he gets to college (which is also typical for this trope in BL from what I've seen).
The age-gap between MC and Hatano-kun is 2 to 5 years(it's shorter rather than longer but I'm too lazy to look for exact numbers). It's a very basic age-gap that is pretty much unnoticeable as they get older. Boy has been crushing for 8 years so a few more to wear off MC's walls doesn't seem impossible.