How old is Hero from OMORI?

Hi, struggling with something here so would appreciate some help haha.

Hero was 15 in Headspace, so I assumed he was 15 when Mari died. This would be a Freshman in highschool, right? Meaning he'd graduate at 18, and start his first year of college at 18. He'd finish his first year of college at 19, and start his second year of college later in the year.

Meaning when the next summer rolls around he's 20, and since the real world-segments take place during summer wouldn't that make him 20 and not 19 like everyone assumed?

My evidence for him being a second year college student is the fact that after rescuing Basil from the lake, if you go back to Kel and Hero's house and interact with the Cookies/Hero sandwiches I believe that Hero mentions it being the same party as last year, implying this is his second time coming home from college. Either way I would appreciate insight on this! Very confused by it.