You should watch Pretty Noona Who Buys Me Food
It is airing in Korea right now and its storyline is so smooth and the chemistry between the main leads is amazing~ (I am seriously addicted to this drama right now) I have watched a lot of noona romance dramas and I am 100% sure that this drama is my #1 from now on.
On the other hand I really recommend I Hear Your Voice, Reply 1988, Angry Mom (although they won't be a couple jisoo was so cute there, and actually it is not included in romance genre), Kenja no Ai (storyline has a revenge factor ), Kimi wa Petto (it has 2 japanese adaptions and 1 korean adaption), Madam Antoine (not a huge fan) , Modern Farmer, Panda and Hedgehog (to be honest I watched this 4-5 years ago so I cannot remember their age gap but if I am not wrong female lead was at least 1-2 years older than Donghae, and the couple was cute), Plus Nine Boys (there are lots of couples one of them is noona-dongsaeng themed), She Was Pretty (good one), Suki na Hito ga Iru Koto (you should definetly watch this~Kento Yamazaki ^_^), Surpluss Princess (not the main couple but they were soooo cute), My Love From Another Star and Hello Monster