3. King 2 Hearts. The leads have a 9 year age gap.

4. Dr. Romantic 2 - Graduated from same class showing that they are the same age in the drama but in real life there is 5 years age gap between the actors.. 

5. Kieta Hatsukoi - Study in the same class in the drama but there is 5 years age gap between the actors. 

6. Let Me Be Your Knight - ages of characters are 4 years apart, actors are 6 years apart

7. Commuting to School Series: Commuter Tram 

8. HIStory2: Crossing the Line

9. While You Were Sleeping

10. Put your head on my shoulders. 


Since I like this topic, I'll pick something similar.. 

10 dramas/movies where the actor in real life is older than the other but in drama he/she acts as younger than the other.

1. What's Wrong with Secretary Kim  - Park Seo Joon is actually 7 years older than Lee Tae Hwan who plays his older brother in the drama 

2. Cute Programmer - the ML's character is older than the FL (I'm pretty sure they are even an age gap couple in the drama), but in real life, the actress who plays the FL is five years older than the lead actor.

3. Sotus - it is not by much, but the ML, Arthit, is the ML, Kongpob's, senior in school by two years. But in real life, the actor who plays Kongpob is a year older than the actor who plays Arthit.

4. TharnType - ML is 19, while his older brother is a few years older. In real life the ML actor is 1 year older.

6. Nabi, My Stepdarling - the FL, Nabi, is two years older than the ML in the drama. However, the actor playing the ML is the older one in real life.

7. The Rebel Princess- FL pays a teenager marrying an older general  in the drama but is 42 IRL. She is 3 years  older than the ML in reality

8. The Untamed - Xiao Zhan (playing Wei WuXian) is 9 years older than Wang YiBo (playing Lan WangJi); in the drama, LWJ is asked to sit in on the studying sessions to give a good example to the newbies, implying he's a bit older than the newcomers. He's also the more mature of all the students. In the novel, they're around the same age, though it's not specifically mentioned which one is the older.

9. A Girl Like Me - FL has a younger brother while the actor who played the brother is 4 years older than actress playing FL.