Schrodinger's Puberty
Until you look inside the box, your younger relative is both a child and an adult. The last time you saw them they were in a Diaper and now they make complete sentences. Then the next time you look, they're starting highschool. Then they're driving. Then they have their own kid. In your own head they're still a child even when you're talking to them about how to manage a mortgage.
Last time I saw Dani, she was starting high school; Then she went through schrodinger's puberty and got married.