Shes saying that because it's not looked we upon a young man with a much older woman. Compared to the opposite which isn't AS bad even tho it's bad. It's just frowned upon to have such an open relationship so that's all she is trying to say "be quiet about the relationship because it doesn't look good" (=・ω・=)
But she needs to consider his feelings too
Tbh what she is saying is valid though, she's under the impression that's he's just a silly boy that will someday leave her when he realizes she's old just like mincheol
This is crazy y'all had sex and all and then talk about "i am not your age" please