I mean, that's hypothetically less invasive than requiring your photo ID, and it's exactly what people have been asking for for stuff like Steam being required to identify a user's age. If the account itself is over 18 years old, the user is. If it had an attached debit card, you can probably assume it's an adult. Etc.
So on one hand, eww privacy. On the other, it's info they already have on you and better than forcing ID.