So guess she would be inhuman, but some sorcerer or body travel is fine too, she should look young, not necessary a little girl young, but at least milf young. Don't care about genre as long as they about 80% time together, some fighting seinen where hero fight with hairy mans more then half a time and just occasionally return to heroine is a no. It may be some nice slice of life or some epic adventure or some economically hard something. Want them to behave more or less adult-like, peeping idiot or facking tsundere is a no. She could be a little tsun-tsun like sensei in Trinity Seven but then mc should balance her perfectly. For some reason think people will recomend Spice and Wolf, already in future read list.
Thank in advance. Sorry for engrish.

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"If you don't remove your hands immediately, I will render you unable to biologically maintain life." - that's how all shoujo must be lol.
"Her breasts, freed from the restraint of a bra, bouncing in tandem creating a beautiful concerto." - Rika

Lets see:
Dance in the Vampire Bund - has the age gap, but I'm not sure how well it fits the rest
Arifureta Shokugyou de Sekai Saikyou (Novel) - the girl shows up after a number of chapters and she is more 300 years older than the male lead; they seem to be together most of the time

The suggested Mahou Tsukai Kurohime might fit somewhat, but

and it kind of starts to suck in the second half.

Spice and Wolf is the winner of this tread and Durarara!! has such a subplot.

Do you know of the movie Harold and Maude ? It is actualy good.

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I also read EU/US comics and am a librarian.
Manga-Masters, My ANN-Lists + Imdb

For the two of you who recommended Spice and Wolf, I take it you didn't read the OP in its entirety.

Shingetsutan Tsukihime perhaps, but I don't remember how old she actually is.
The Voynich Hotel too, but you're already reading that.