Noona-romance for the win! (drama version)

Main couple age gap: 20 years (40/20)

Main couple age gap: 10 years (28/18)

Main couple age gap: 6 years (?) (35/29?) I don't know if Joon Hee's age was explicitly said in the drama.

Main couple age gap: 6 years (at the beginning of the drama 24/18)

Main couple age gap: 7 years (?) (33/26?) I don't know if  Jin Se's age was explicitly said in the drama.

Main couple age gap: 14 years (39/25)

Main couple age gap: 13 years (40/27)

Main couple age gap: 10 years (32/22)

Secondary couple age gap: 7 years (35/28)

Main couple age gap: 10 years (32/22)

Main couple age gap: 7 years (33/26)

Main couple age gap: 16 years (32/16)

Main couple age gap: 23 years (43/20)

Main couple age gap: 10 years (37/27)

Main couple age gap: 10 years (36/26)

Episode 10 => Main couple age gap: around 15 years (40/25)  

Main couple age gap: 7 years (31/24)

Main couple age gap: 10 years (38/28)

Main couple age gap: 18 years (40/22)

One of the main couples age gap: 10 years (35/25)

Secondary couple age gap: 6 years (27/21)

Main couple age gap: 5 years (24/19)

Main couple age gap: 8 years (28/20)

Main couple age gap: 6 years (26/20)

/Their age gap is not an issue in the drama and no one even brings it up./

Main couple age gap: I think it's at least 5 years (42/37)

/It's a melodrama with heavy and toxic emotions/

Main couple age gap: 15 years (?) (34?/19)  I don't know if Yoon Soo 's age was explicitly said in the drama.

Main couple age gap: 8 years (29/21)

It's not really a romance per se but still ML is at least 6 years younger than FL (38/32)

Main couple age gap: 8 years (27/19)

Main couple age gap: 8 years (30/22)

Main couple age gap: 7 years (34/27)

Secondary couple age gap: at least 5 years (31/2?) Choi Wi Jin's age was not explicitly stated.

Main couple age gap: around 4-5 years (32?/27?)

Main couple age gap: 17 years (32/15)

Main couple age gap: 10 years (?) (34/24?)

One of couples age gap: 6 years (30/24)

Main couple age gap: 4 years (24/20)

Main couple age gap: 7 years (33/26)

Main couple age gap: at least 5 years (?) (33?/28?) there is also a background couple with a similar age gap

Main couple age gap: around 5 years (34?/29)

/Their age gap is not an issue in the drama and no one even brings it up./

Secondary couple age gap: 6 years (27?/21?)

Main couple age gap: 4 years  (29/25) there is also a secondary couple with 10 years age gap (33/23)

Two background couples: 1) around 8 years (?) (30?/22) - it's cute but unrequired; 2) around 15 years (?) (45?/30?)

/The romance in this drama is the slowest of the slow burns/

Main couple age gap: around 8 years (33?/25)

Main couple age gap: 4 years (30/26)

One of the couples age gap: around 7 years (35/28)

The main couple's age gap: around 20 years (40+/20+) -  it's cute but unrequited, because of the body swap.

Very slight romance with vague open-ending; age gap: around 14 years: (40?/26?)

Main couple age gap: 5 years (27/22)

Episode 2 => Main couple age gap: 8 years (32/24) 

Main couple age gap: 15 years (39/24)

Main couple age gap: around 5 years (?) (33?/28?) there is also a background couple with around 10 years age gap (35?/25)

One of the main couples age gap: 12 years (38/26)

Main couple age gap: 7 years (33/26)

Main couple age gap: around 4-5 years (32/27?)

Main couple age gap: 6 years (31/25)

Main couple age gap: 7 years (31/24)

Main couple age gap: at least 6 years (23?/17?)

Main couple age gap: 4 years (26/22)

Main couple's age gap: 6 years (35/29) there is also a background couple with a 3-year age gap (32/29)

Main couple age gap: 10 years (32/22)

Main couple age gap: 9 years (37/28)

Main couple age gap: 10 years (25/15)

Main couple age gap: 9 years (30/21)

Main couple age gap: 10 years (35/25)

One of the background couple's age gap: at least 25 years (51?/26)

/I'm assuming she's in 50-55 range because she has a 13-year-old son but looks around 60 years old/

Secondary couple age gap: around 4-5 years (29?/24?)

/There is also a background couple with a similar age gap (something like 25/21)/

Episode 1 => Main couple age gap: around 7 years (30/23)  

Background couple (unrequited love) age gap: around 5 years (27/22)

Main couple age gap: 4 years (30/26)

Main couple age gap: around 6 years (36?/30?)

Main couple age gap: around 5 years (36?/31)

Secondary couple age gap: 8 years (44/36)

Main couple age gap: 5 years (33/28)

Main couple age gap: at least 20 years (?) (42?/22?) there is also a background couple with a similar age gap

Main couple age gap: 7 years (32/25)

Main couple age gap: 6 years (39/33)

Main couple age gap: around 7 years  (33(?)/26)

Main couple age gap: 10 years (35/25)

Main couple age gap: 7 years (32/25)

One of the main couples age gap: 12 years (33?/21?) I don't know if they said how old are they. 

Main couple age gap: around 11 years (32?/21)

Main couple age gap: 4 years (31/27)

Main couple age gap: 9 years (30/21)

Main couple: age gap 17 years (45/28)
Background couple: age gap 14 years (40/26)

Main couple age gap: 14 years (33/19)

Main couple age gap: 8 years (33/25)

Main couple age gap: 5 years (33/28) /they had an affair 10 years earlier/

Main couple age gap: 15 years (38/23)

Main couple age gap: 6 years (36/30)

Main couple age gap: 12 years (31/19)

Main couple age gap: 6 years (29/23)

Main couple age gap: around 15 years (40?/25?)

Background couple: age gap around 25 years (60?/35?)

Main couple age gap: 6 years (34/28)

Main couple age gap: around 5 years (?) (33?/28?) The ages are my guess based on other characters' ages.

Secondary couple age gap: around 20 years (45?/23?)

/I don't think their ages were clearly stated. The guy graduates from a university near the end of the drama and a woman is a mother of FL. /

Main couple age gap: 4 years (27/23)

Main couple age gap: 10 years (30/20)

Main couple age gap: around 5 years (I don't know how old is Su Ban Xia)  (27?/22)

Main couple age gap: 8 years (33/25)