Chugakusei Nikki - Jdrama; about a teacher and student relationship that spans over 8 years. The drama is on the melo side but has a good ending.
Flower Boy Ramen Shop - Kdrama; Older woman, younger man / teacher and student relationship. It's a romcom.
High School King of Savvy - Kdrama; Older woman, younger man. The woman is working age in her mid 20s. The boy is a senior in Highschool. Cute romcom.
I Hear Your Voice - Kdrama; Another older woman and younger man. They met when she was a middle schooler and he was just a kid. The story is like a suspense, mystery theme. Great cast and very good chemistry.
My Ahjussi - Kdrama; Older man, younger woman (IU). It's a melo, which focuses on life, friendship & marriage. (Not very romantic)
Morrasoom Sawat - Thai Lakorn; Older man (Weir), younger woman (Preaw). I remember really liking it. P'ek is a mafia, and n'ek at one point was "sold" to him? (either that or she was used a collateral), it's a slap/kiss type.
None of these were from the 90s, but they're pretty good if you are looking for this type of genre. I can add more later if anyone is interested. A lot from Jdrama/Jmovies. They seem to like making this type of plot, lol