Who Are You? (2008) is one of my absolute favorite age gap dramas that isn’t rly widely known of or mentioned. It’s not like, huge, but she just graduated from hs (so 18+/19) & he’s almost 30 I think so definitely significant.
A lot of historical dramas like Dong Yi have a pretty significant age gap between leads, but it’s not the center of attention in their couple because circumstances were different and he’s kind of, uh, the king lol.
Also, a previous poster mentioned An Incurable Case of Love already but I’ll mention I found it really cute and honestly couples of that sort aren’t really ones that I consider unrealistic, like at all.