I'll try to add to the ones posted before without repeating. I'll give the leads real ages.
Mirror of the Witch- Yoon Shi Yoon is 14 years older than Kim Sae Ron
Beautiful Gong Shim- Nam Goong Min is 15 years older than Bang Mi Ah
Behind Your Smile- Chang Marcus is 7 years older than Liu Eugene, I add it because in the drama she acts much younger so is very noticeable.
Cheese in The Trap- Park Hae Jin is 8 years older than Kim Go Eun, although in the drama they are supposed to be around 4 years apart (he was in the military and came back to university) it still noticeable.
High-end Crush- Jung Il Woo is 7 years older than Jin Se Yeon and in the drama he is supposed to be older too.
Introverted Boss- Yeon Woo Jin is 10 years older than Park Hye Soo
Itazura na Kiss- Love in Tokyo- Furukawa Yuki is 10 years older than Miki Honoka, even though in the drama they are supposed to be about the same age.
Kill Me, Heal Me- Ji Sung is 8 years older than Hwang Jung Eum, they also star together in Secret Love.
Love in the Moonlight- Park Bo Gum is 6 years older than Kim Yoo Jung, very noticeable because she was 16 at the time of the drama.
Marriage, Not Dating- Yeon Woo Jin is 8 years older than Han Groo.
My Little Bride- Korean movie about arranged marriage, she's a high school student, he is her teacher. In real life Kim Rae Won is 6 years older than Moon Geun Young.
Close Range Love- Another teacher-student relationship movie. Yamashita Tomohisa (Yamapi) is 11 years older than Komatsu Nana.
My Secret Romance- Sung Hoon is 7 years older than Song Ji Eun.
Reply 1994- Jung Woo is 10 years older and Yoo Yeon Seok is 6 years older than Go Ah Ra. You need to figure out who is the lead in the drama.