http://www.mangago.zone/read-manga/secretary_and_childe/
They are 18 and 30 in the main story (which will come up soon)
younger seme:
http://www.mangago.zone/read-manga/on_doorstep/
http://www.mangago.zone/read-manga/lover_boy_lezhin/?pgJt
http://www.mangago.zone/read-manga/boku_no_omawarisan/
http://www.mangago.zone/read-manga/ecchi_suru_nara_papa_ana_de/ *with adoptive dad*
older seme:
http://www.mangago.zone/read-manga/koi_ga_ochitara/
http://www.mangago.zone/read-manga/kono_ore_ga_ahe_goe_nante_dasu_wake_ga_nai/
http://www.mangago.zone/read-manga/liebling/
age gap but toxic:
http://www.mangago.zone/read-manga/body_complex/
http://www.mangago.zone/read-manga/unfather/
Age gap but they're both over 18 please. Something like this:
http://www.mangago.zone/read-manga/fushidara_na_hanatsumi_otoko/