My sister was like that: looked like my mother’s sister and never resembled my father until suddenly, at about age 30, she did. I think it’s a genetic timeline that causes certain traits to kick in at some point—weight gain, let’s say, or vision change, or arthritis, or how the shape of the face ages—that might not have shown up before. It makes sense to me that not all the traits you’re ever going to have are manifest at birth (even if there’s wiring for them) and that the timing of when they appear may have a hereditary component. But I’m just theorizing. This isn’t my field of study.