Viewfinder - Seme is 35, uke is 23 ... I don't know if that is enough of a gap for you though. (or maybe you have read it already too ...)
Mujihi na Otoko - there is a professor/student 'pairing' but the teacher is not the main character
Another one, that I thought of after seeing "any genre" is Kuroshitsuji ...
i love yaoi/shonen so im just list some of my fav,hope u like it
^_=
http://www.mangago.me/read-manga/love_mode/story
from this author:(there lots ojisan pairing)
http://www.mangago.me/r/l_search/?name=+NAONO+Bohra+
thank u so much ^_^
can someone please recommend me a manga with an older and a younger partner. Someone like in their 30's or 40's. Like the difference between kageyama and kuga from don't stay gold. I love it when the younger one calls the older one old man :D It can be any genre. I love ojisans! >.<