I think they're 17 or 18. Cuz mitsuki getting his dream job/college paper to fill from his teach, him getting his hair dye and ears pierced, this wouldn't be allowed if he was a young teen, esp with no present parent. Everything around them and their characters just feels like they're late teens atleast. I hope it's a mistranslation that they're 15 and maybe the author's mistaken 😭
I'm all BI myself~~ 💗💜💙