They actually not an adult when they died, they’re in their teen (if I remember correctly). I also think that when they reincarnated their minds merge to the current self, adaptation. So, I don’t see any fault at their characterization, they still sometimes shows ‘older than they look’ vibe (?). Again, they reborn as baby.
Edit: I checked the 1st chapter, they died at 18, still in teen. Then 2 years blank, then reborn or reincarnates when they were 20y/o but that was blank period (they still death at this 2years period)