His whole MC into people outside his age range logic is very flawed like most other people who try to think that is an actual argument. The MC DIED first and then was reborn in a new world. Whatever age he was is reset the moment he died and was reborn in a new life. So what if he keeps his memory and personality, that is not what determines their age, and the idea they have to lose them just to be considered young, is completely dumb and highly flawed in every way. If the MC didn't die first and only switched to another body, then maybe you could have a case, but even that one is flimsy at best because MC has no control over it and it's not like he has any reason to believe he'll ever get his still functioning older body back, so it's ridiculous to think he should be going after adults in that life when he is in fact a kid again. It's really not hard to understand, but people want to do these crazy mental gymnastic and think they have any sort of moral high ground, when they don't.