Does your objection extend to fics that age him up (either having him sacrificed later/post DoTO by a few years) or do you consider the fact that the source character is underaged to be the problem? In addition, do you expect so see character growth/a different personality in the story that somehow reflects that additional three+ years passing before deeming it to have solved the problem?
that depends entirely on the context of why he’s aged up. there’s nothing wrong with writing the outsider a few years older if your only reason was an exploration of how his personality would change if he became human and started aging again or something else of that nature. i’m actually writing a fic of my own about that exact thing!
but if you’re aging up a minor just for the purposes of pairing that minor up with an adult, that’s OBVIOUSLY bad.
i don’t think it’s a problem that the outsider is fifteen, and it’s not something that can be “solved” or really needs to be solved by future games (although i would be really interested to see what the canon take on his future would be). the problem has always been and will always be the fans who don’t check his age before producing content that puts him in relationships with adults or, worse, the ones who do look it up and still make that content anyway.
(not to say that the canon is perfect about it either, just based on the fact that the outsider’s character model does not resemble a normal teenager at all or the fact that some random npcs say some decidedly nsfw things about him in canon, but that’s not an excuse for the fans to sexualize this void teenager)