(one) is old enough to be (someone's) (parent/grandparent)

One is old enough to be someone else's parent or grandparent. Used in reference to two people who are or may become romantic or sexual partners, especially to indicate that the speaker thinks such a pairing would be inappropriate. He's going out with Mary from the legal department? But she's old enough to be his mother! I don't feel comfortable with this. I'm old enough to be your grandfather, for heaven's sake! I highly doubt Ron and Elise are going out. For one thing, he's old enough to be her father. For another, they're always squabbling with each other.