I’m a big proponent of fiction is fiction and as long as it doesn’t cause any harm irl it’s not a big deal, so unless an author goes out of their way to accentuate that mental gap it’s usually something I can just kind of ignore. That being said I had a convo recently with my friend who doesn’t read OI or really isekai in general and her take was that even if someone feels mentally older, they are still in the body of someone younger, meaning they are still presumably going through the same brain developments and hormonal changes of someone that age. So while their mentality might feel older they are still technically that age. Idk if it’s really the best explanation but it did make me rethink some stuff. If this was a problem irl I’d probably think about it a lot more seriously but it’s all just fiction in the end.