From her reaction is clear that MC resembles someone she loved in the past, that is probably dead.
Her welcome was so serious because she maybe thought he was the reincarnation after Itsuki sent her his info, but noticing the behaviour and all switched to happy-go-lucky lady on the spot.
giving ther ages is mot likely no reincarnation scenraio, he just resemble the unknown dude.
I think the age difference is 13-14 years, big, but still acceptable, even more in a manga where people age differently.
I dislike how he will call her "mom" instead of landlord or her actual name, guess she was hoping that he would get embarassed and call her Hana, but he i super serious and gone with the other, dumb.