http://www.mangago.me/read-manga/yamada_to_shounen/
http://www.mangago.me/read-manga/unfather/ (fucked up tho lol)
http://www.mangago.me/read-manga/hana_wa_saku_ka/
http://www.mangago.me/read-manga/castle_mango/
http://www.mangago.me/read-manga/not_equal/ (incest but a rly good story)
http://www.mangago.me/read-manga/the_priests_chart/
http://www.mangago.me/read-manga/bright_light_sprout/
15 yr age gap. http://www.mangago.me/read-manga/kokoro_kanda_neko/
Yesss seconding yamada to shounen!
Others include:
http://www.mangago.me/read-manga/kirei_ni_shiteyo/
(17 and 39)
http://www.mangago.me/read-manga/boku_no_omawarisan/
(20 something and 40)
http://www.mangago.me/read-manga/mujaki_na_wanko_to_nekokaburi/
(21 and 40)
http://www.mangago.me/read-manga/young_good_boyfriend/
(16?? and 37) has a sequel where they grow old together
http://www.mangago.me/read-manga/young_good_boyfriend/
http://www.mangago.me/read-manga/himono_tokodoki_shanairen_ai/
(20 something and 40 something)
For straight, um...
http://www.mangago.me/read-manga/musume_no_tomodachi/
I just read this and it seems pretty realistic so far :v
(father has a daughter the same age as FL)
More teacher x student:
http://www.mangago.me/read-manga/shiota_sensei_to_amai_chan/
http://www.mangago.me/read-manga/sensei_can_t_teach_me_about_love/
and...
http://www.mangago.me/read-manga/koi_to_yobu_niwa_kimochi_warui/
high schooler and a late 20s person
for gl
http://www.mangago.me/read-manga/beloved_jaeliu/
16 and someone in their mid thirties i think
http://www.mangago.me/read-manga/sayonara_alpha/
forgot this lol, omegaverse
there's also the side pairing in love is an illusion, 12 yr gap
http://www.mangago.me/read-manga/love_is_an_illusion/?av=2
Looking for a big " age gap " romance manga.
I tried looking through the tag, but the age gap are too small.
I'm mostly looking for something similar to: Usagi drop and After the Rain.