@Tych watashi is also used by adults. when growing up boys usually use boku and girls usually use watashi but around near the end of highschool boys would start switching to watashi as that is considered a more proper way to say I while boku is looked as childish. he was the great sage in his past life so he probably used watshi as its more dignified then boku but because he was reincarnated he had to switch back to boku as using watashi as a male child is not normal
His common sense is probably warped beyond repair, that's why he does not realize that everything he does is basically raising flags for the Hero route...
I want a peaceful life... so I'll just combine some advantages of my hero life and my sage life and become an adventurer... who within the hour has already cause a huge disturbance... makes sense... followed.
For someone that has lives 2 life, he is incredibly clumsy at how to hide his identity to get the job done. I don't believe he is an adult with 2 life 😅