I forgot to say this last chapter about the comments of "he is 10" but forgot to cause i was focused on the MS Paint bit. She knows he was reincarnated, therefore she knows he isn't 10 on the inside. If she had been treating/thinking of him as if he was really 10 then yeah I would say it is beyond creepy, but to her he can make decisions as an adult since he is an older person put in a child's body. Now as soon as she starts getting frisky with his dingle dong all that goes out the window and "he is 10" is a big issue.
Math time:
Looking back at chapter 1 it seems like mc was wearing some kind of school uniform and im pretty sure that style is mainly for middle schoolers. So if we assume he was 14 when he got killed by the evil god and then he has lived 10 years in this world then he would technically be 24 which is plenty adult to be deciding if you want to date some girl.
Technically it might be appropriate to say that the first 5 years of his new life don't count since he only regained his memories after he was 5 and that he is only 19. However, since idk the how regaining 14 years of memories when you are 5 would affect a person's emotional maturity I am just going to assume that whatever age his mind is is closer to 24 than 19.
As for her, I can't remember if they said how old she was, but I'mm pretty sure she is said to either have a really long life span or have eternal youth. So I will just judge her based on how old I think she is supposed to look until one of you people tell me how exactly pseudo immortality affects a person's emotional development. She appears to be somewhere older than 20 as she doesn't appear to be a child, and younger than 40 since she doesn't show any signs of being old. So I will go with 25-32.
So a probably close to 24 y/o with a probably close to 30 y/o.
I say it isn't too weird as long as his willies stays out of the equation.