I'm looking for a story where the main pairing is between older female and younger male, and they must be acquintanced for more than a moment first (so I'm not looking for boy meet girl and suddenly fall in love type of stories. I prefer a teacher x student, a boy with his senpai/auntie next door etc where they had relationship before).
Most likely not what you're looking for, but "Gate" immediately comes to mind with your request. After all, with Rory having an age of 961 years, she's definitely the oldest of the pair.
This aren't novels but... -Age Matters -The Young Master and The Maid -My Lv999 Love for Yamada -The results from when I time leaped to my second year of high school and confessed to the teacher I liked that time (This one has a novel version too) -The Teacher can not tell me Love -Net-Juu no Susume (The anime is far ahead of the manga translations) -Wotaku Girl and High school Boy -The Reborn Witch Foretells Destruction -The two gets by Tomorrow too, more or less -Kawaii joushi wo komarasetai Are all titles that comes to mind... The other ones are just fluffy Romcom without much plot in it so I didn't listed them but if you want to try out just look for me.
Spice and Wolf condor heroes trilogy: legend of condor heroes, return of condor heroes, The Heaven Sword and the Dragon Sabre Mushoku Tensei (polygamy though)