Some different recommendations! All with older man/younger person.
Koukou kyoushi 2003 (j-drama, male teacher around 30 or late 20's/female high school student)
Koi (j-special, male teacher/female university student/wife, with a twist)
Mitsu no aji (j-drama, girl and her not blood-related uncle)
The Painter of the Wind (k-drama, crossdressing female young art student/male teacher - historical)
Utopians (HK movie, male university student/male student - there are sex scenes)
My Rainy Days (j-movie, university teacher/high school girl)
Beethoven Virus (k-drama, male conductor/female orchestra member, not huge age gap but still)
Valid Love (k-drama, male teacher/female high school student, but most of the story is later when she's older but the age gap is still there of course!)
Iki mo dekinai natsu (j-drama, 40-something man/19-year-old girl, but the romance is mostly between the lines)
Battle of Changsha (c-drama, man around 30/16-year-old girl, amazing drama)