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I'm looking for kdramas where the female lead is much younger than the male lead. Maybe she's in high school and he's older or like in her twenties.
I've already seen:
My Princess
My Little Bride
Doctors
Bring it on, Ghost
Currently watching Goblin
Prime Minister and I
Marriage Contract
Hello School Girl
Valid Love
Coffee Prince
Remember, Korean Dramas only please :) I want a feel for the older protective male figure. Kinda like the vibes in the show Goblin
Not sure who you would consider the lead, precisely, but Babysitter has a college-age girl in relationships with two men in their forties

Also Painter of the Wind, but throw in some cross-dressing and not a lot of actual romance, haha
Not sure if these are what you're looking for but age is a factor in these:
Bad Family
Dating Agency: Cyrano
Sweet 18 - she's in high school here
The Best Lee Soon Shin
The Eccentric Daughter-in-Law
Valid Love
They meet when she's in high school (he's a teacher) but don't get together until later. 
coffee prince? Hello schoolgirl (movie)?
Mr. Back
Entertainer
I Love Lee Tae Ri
Witch's Romance
Romance (http://mydramalist.com/1900-romance-2002)
Land of Rain
Its Okay, That's Love (side couple)
Hello Schoolgirl - He's 30 and she is 18.