Isn't Haru underage? Or do such laws not exist in their world?

It's an omegaverse, so it's pretty standarized that once an Omega gets his or her first year they're allowed to mate

He seems to be in highschool. I'm assuming that he's in his last year of highschool. Since Japan has 3 yrs middle school and 3 years highschool, he's most likely 18 if he's in 3rd yr hs. But yeah, what springrol said is likely the most accurate explanation.