The younger man is the seme--
http://www.mangago.zone/read-manga/ren_ai_nenrei/
you can choose from my younger seme list, i did mention the age and made individual review of each http://www.mangago.zone/home/mangalist/43695/?filter=&page=2
http://www.mangago.zone/read-manga/zurui_otoko_crafty_man/
http://www.mangago.zone/read-manga/hada_no_ue_no_renai_jijou/ (naono has a lot of that but i don't remember which manga has that kind of concept )
http://www.mangago.zone/read-manga/sumanai_masumi_kun/
http://www.mangago.zone/read-manga/kimi_to_date/
http://www.mangago.zone/read-manga/akuma_to_issho/ 3-5.5
http://www.mangago.zone/read-manga/usagi_otoko_tora_otoko/ vol1 from pg145 & vol2 from pg155
http://www.mangago.zone/read-manga/sora_to_hara/
http://www.mangago.zone/read-manga/occupation_to_beloved/
( http://www.mangago.zone/home/mangalist/70326/ more older guys xD)
I would love some yaoi suggestions where a younger man pursues an unconfident older man (15+ year age gap, but no shotacon please). I just read the first few chapters of "Because of You," and I just love that type of scenario. Thanks !!