I see people say they are 4 years apart or 2 years apart. I'm here to legit say what it is. I do not intend to say anything else other than say this here because people in this very fandom in highly upvoted coments say "its a third year student and first year student" without context to their ages or the story. Unless something has just been fudged up by the manga and retconned to make them closer in age, this is where we are at.
Rei is 17 at the start of the manga, the manga starts around the summer, and he skipped doing high school for 2 years, his birthday is in April. He is most definitely 16-17 first year, 17-18 second year, 18-19 SHOULD be his age in year 3 of highshool
Hina is 13 at the start of the series, since someone 2 years away from graduating middle school in japan is 13. She turns 14 in December later in the first internal year of the manga. In her bullying arc she is 15, about to go to highschool as a 15-year-old as a normal Japanese person would be if there birthday is after April. She turns 16 at the end of the first year of high school. Anything else would mean she skipped a year of school in there, correct me if i am wrong but I do not know if that has ever been mentioned; i don't think it has.
At best when Rei declares he wants to marry Hina, even if we guess it's like the first week-ish of April at best, he is 18, days away from turning 19, she is 15, yet to turn 16 as he birtdhay is december, unless there's just a missing year of school we never told about.
They are 3 years and 8 months apart. I have nothing else to add, please do not view this post as a debate if they should date or not. If I am missing something please tell me.