Oh, like the movie "Harold & Maude"? I love huge age gaps like that! I mean, they can be kind of sweet, if done right.
I'm presuming you don't want immortal romances, with vampires or other non-agers, but in case you DO, massive age differences can be found in
Immortal Rain,
Meteor Methuselah,
Blood Alone (even though the technically "older" vampire looks like a child),
Vassalord (if you're okay with gayness) and a ton of others that I might recommend if you're not averse to this kind of age-gap.
The only large "human" age-gaps I can think of right now are in
Ristorante Paradiso and (possibly) in
Gente. The mangaka does LOVELY oyajis, truly droolworthy and cute, even in his/her yaoi work (e.g.
Kuma to Interi, which also features some large age-gaps). There are more massive age-gaps in yaoi than in any other genre, so let me know if you're okay with BL and I can rec you some titles in that genre, too.
