How far was the age gap in Survive as the Hero's Wife manhwa? I don't remember. You can check it out, you'll probably find out in the first few chapters. And also try this manga: SSB - Chou Seisyun Kyoudai Though same here, I don't remember how far the age gap was. Well, you need only read the first chapter to find out. I don't have the will to do it, but maybe your determination to read something with this troupe might pull you through. 'tis truly a difficult task but believe in yourself and you'll be able to do it. Edit: Sorry, I get a bit dramatic when I'm tired. (Believe it or not, it's worse in real life. I'm somehow more annoying when I'm tired and out of energy.)
It's adapted into anime, but the manga still ongoing. Maybe try the Emperor's Treasure too, the ML is younger but it's always subtle since he's taller than MC.
Sincerely I became the duke's maid, young master's maid, princess Wei Yang (I think the male lead is younger but im not very sure of it) and nei and souta's petite something( this is very shoutacon tho, like hes elementary shes like high school)
Sorry, I can't access my opera bookmark thus I can only give you three webtoon that fits your criteria When the Tangent Meets The Innocent sin (this one is smut) Re: Marina
Mangas/animes: Otoyomegatari (at least vol 1 & 2) Game Over - Fuka Mizutani Onegai, Sensei! Aa! Megami-sama (technically, Bell & other goddesses are older than Keiichi) Tenchi Muyou (same reason as above) & Tenchi GXP