i personally would still recommend Sweet 18...10 year diff between characters, even though only 4 years between them in real life.    Its a  forced-cohabitation drama which I love. 

I'd also add the movie Otoko no Isshou / Her Granddaughter to the list! I don't recall the character's ages but Toyokawa Etsushi (male lead) is 26 years older than Eikura Nana (female lead). It's a rather slow film but Toyokawa's character had a great sense of humour, and I found the love story to be really sensitive and sweet. (Also introduced me to the term "fossil hunter" for ladies who are into older men lol!)