Did we know exactly how old Phainon was when he left Aedes Elysiae in canon? A reddit post mentioned he is a teen between 13 to 16yo.
I know your slay the princess fic has it's own lore but I was curious if you made him 8yo because of canon or just created your own lore age for him in your fic. (Yes I am not going stop thinking about your fic anytime soon).
There are no mentions of possible ages in Phainon's story besides that one readable where he says he got scolded by his parents one time for leaving wheat out in the rain when he was eight. Aside from that, in the Amphoreus' Saga of Heroes entries, they refer to him as the “smiling youth” quite frequently.
That being said, it's not possible to get exact ages either, since the Eternal Recurrences were all a little different from each other. And how old Phainon was when he got to Okhema depends on when Aedes Elysiae got destroyed/was destroyed by the Flame Reaver. In Cycle#0 at least, he seems to be a teen, judging by the CGs.
While it is true that he took 10 years to graduate, we also have to remember that the Grove of Epiphany doesn't function like our education systems. As in, you don't have to be a certain age and only that age to study there. For example, Castorice, despite being something like a thousand years old, studied alongside Phainon.