I guarantee you there are lots of sleeper accounts out there that have made "a purchase", which could be less than a dollar. It definitely helps to require any purchase at all, but if they were trying to be semi-thorough that requirement would look more like $100 in purchases.
>Limit one signup per household. We will use payment method, shipping address, and other information to eliminate multiple entries."
It's not just an account with an age, they have to have made a purchase. And shipping address + payment info also help eliminate duplicates.