bl w age gap? preferably w older seme

You mean like shotacon?

Here’s Liebling! if you want not shotacon
https://www.mangago.me/read-manga/liebling/

You can also loo through the tag:
https://www.mangago.me/tag/age+gap/

https://www.mangago.me/read-manga/one_step_from_hell/ (Seme not human)
https://www.mangago.me/read-manga/on_or_off/
https://www.mangago.me/read-manga/shinigami_wa_korosenai/ (Seme not human)
https://www.mangago.me/read-manga/missing_code/ (I think he’s older not sure, but he looks older...)
https://www.mangago.me/read-manga/doutei_sama_no_omou_ga_mama/
https://www.mangago.me/read-manga/musebenake_junjou/
https://www.mangago.me/read-manga/if_you_tell_me_to_do_it_i_ll_do_it/
https://www.mangago.me/read-manga/down_and_dirty/
https://www.mangago.me/read-manga/natsumichi_drive/(Cousins)
https://www.mangago.me/read-manga/in_my_closet/
https://www.mangago.me/read-manga/our_companionship/
https://www.mangago.me/read-manga/sugar_dog_life/
https://www.mangago.me/read-manga/kami_sama_no_iutoori_moegi_yuu/(Seme not human)
https://www.mangago.me/read-manga/gun_and_heaven/
https://www.mangago.me/read-manga/royal_servant/
https://www.mangago.me/read-manga/glow/(Shounen ai)
https://www.mangago.me/read-manga/my_blood_and_soul_to_you/

https://www.mangago.me/read-manga/koi_ga_ochitara/
https://www.mangago.me/read-manga/how_sweet_is_a_sugar_daddy/
https://www.mangago.me/read-manga/the_crows_marriage/
https://www.mangago.me/read-manga/i_m_no_match_for_him/

A shounen ai https://www.mangago.me/read-manga/otoko_yamome_mo_hana_ha_saku that was great, or just check my list though it’s both older uke and seme https://www.mangago.me/home/mangalist/1004497/