Hiii!!
Well, my new obsession in asian dramas is the ahjussi love (it's like the noona love, but here, the older one is the male character, I mean the age gap between the 2 main characters it's pretty noticeable, it's not by 1-2 years, it's like 5-10 years). But it doesn't seem like there are a lot of korean dramas like this, so I was wondering if you guys knew any? It can be japanese or taiwanese too, I don't mind at all.
So you guys have like a reference of what I'm looking for, it's dramas like:
-Sweet 18
-I Am Sam
-Kou Kou Kyoushi 
-Little Mom Scandal
Hope you can help me!! Thanks!
(Sorry if my english is a bit weird;$)

Have u already watched My Spring Days? It's the most beautiful Ahjussi Romance i've seen! With Sooyoung from SNSD. 
My Love Eun Dong has a 4 year age gap. If I remember more I'll post here.
The Painter of the Wind has exactly what you're looking for.

I think Take Care of the Young Lady also fits with this theme, even though they're both adults, but the female lead acts a little bit childish and the male lead feels like is somewhat older. There's an 11 years gap between them in real life too, so that also helps.

And I second My Love Eun Dong~ when they first met, he was in high school and she was still in grade school; I have to admit that it was a bit awkward at the beginning ><

If you want to see a japanese drama, there's also Himitsu no Kankei ~Sensei wa Doukyonin~ which contains a romantic relationship between a teacher and a student, but it's light-hearted.
I second A Gentleman's Dignity (although the actual "ahjussi love" as you say is only a secondary story line) , and will add When A Man Loves, where the difference is between 10 and 15 years, I can't recall exactly. The characters often refer to their gap and she teases him for being older, but I'll warn you that many people disliked this drama because of the female lead, so you shouldn't jump into it without checking out a few reviews etc.

There's also Reply 1997, where there's a love triangle and there's an "ahjussi love" involved in there -- but the outcome is only revealed in the last episodes, so I won't spoil it for you :p It's also a great drama!

Also there are all those supernatural dramas where the male lead is technically older as he has lived for centuries, but it doesn't show -- I don't know if that would fit. An example would be Scholar That Walks The Night, or You Who Came From The Stars.

Good luck!
Perfect Neighbor ( an older drama, but I liked it)
The Prime minister and I ( some like it, some don't... I loved it)
Coffee Prince.... (Not a huge age gap) 
Blade Man ( I just realized I didn't finish it myself)
Mr. Baek ( didn't finish that one either)
Dating Agency Cyrano. The actor and actress playing the leading roles have a 16 years age gap 
I also think take care of the young lady fits your request. 
Another one i a korean movie miss granny. 
Thank You for all the suggestions, guys!! <3