this is confusingly written. do you mean (1) an age gap of 15-20 years or (2) an age gap story about a 15 year old and a 20 year old?
**EDIT: SO, now that I know that the answer was 1, here are some recs!
--> Koh-Boku & Suki ni Nacchau yo.
--> Boku no Omawarisan & Mujaki na Wanko to Nekokaburi (pretty much everything by Niyama is age-gap)
--> Page......
