I think it's fascinating that there are so many age gap romances in Dramaland! I like this theme, for some reason. I have only seen a few, and I know that there are many recommendations here already, but I want to mention the titles I enjoyed, because everybody has their own taste.
Kimi wa petto - the woman is older... she's still young by my standards (not yet 30) but the age gap is like 10 years. It's a really cuuuute romance and one of my favourite dramas. (Not just because of the age gap romance, more because I can sort of relate to the main character... The theme of the story are how to fight loneliness, finding confidence and a place where you can be your true self.)
Last Cinderella - Older woman (she doesn't look "old" but people say it to her all the time) and a much younger man. In this drama, the age gap is significant and an important plot point. Some of the themes are (female) friendship, being strong enough to stay true to yourself.
Mitsu no aji - The man is 10 years older... in this drama, the age gap isn't the big issue, but the fact that the characters are biologically related.
Koukou kyoushi 2003 - Male teacher and female student. I loved this drama even though most of the characters are morally gray at best and more than a little crazy...
Iki mo dekinai natsu - I'm currently watching this, only a few more episodes left to watch and I'm in love with this drama. The man is around 20 years older than the female lead and they look so good together. So far, they haven't even talked about their ages in terms of "age gap", and romance isn't the main focus of the drama (even though the MDL synopsis says so). It's more like between the lines, at least up to where I last left it, and sometimes I prefer it that way. It's slow-paced but not boring!
Starman ~ Kono hoshi no koi- Older woman/younger man. It's about family mostly. The romance is sweet, I enjoyed the drama (woman finds a man with amnesia and pretends he's her husband) even though not all episodes were great.
