Younger :
Minnie
Same age :
Yuqi
Shuhua
Older :
Soyeon
Miyeon
Soojin
Minnie: Minnie just strikes me as the type of person that wouldn't care much about age. The person she's dating shouldn't be too young though.
Yuqi & Shuhua: Since they're still quite young I think they would be more comfortable with dating someone their own age.
Soyeon, Miyeon & Soojin: Okay, don't ask me why but these 3 just give me the vibe that they would prefer to date someone older. Soyeon and Soojin would probably like a mature s/o therefore prefer someone older and I feel like Miyeon would like for her s/o to take care of her.