> They could produce their 21-year-old younger sister, who has government ID (yeah, I know, that is a resort to government ID).
1. How would you verify that's actually their relative and not a friend or stranger?
2. How do you verify that they are in fact the older sibling and not just saying they are?
They could produce their parents, or any witness of their birth.
They could (if they were born wealthy) produce a hallmarked silver spoon engraved with their name. Not proof, but persuasive.
They could produce their 21-year-old younger sister, who has government ID (yeah, I know, that is a resort to government ID).
Best of all: they could produce a birth certificate, signed by a doctor (not itself government ID, just a prerequisite to getting a government ID).