Spamming account creation won't work, because accounts need to have been created in April or earlier. They also verify address, payment method etc. to reduce double-dipping.
Pre-creating "sleeper" accounts is a common way of circumventing this, though it does require a degree of long term thinking/planning.
It's likely they are weighting accounts that have a lot of (recent) game activity and game purchases as well. Plus, they have access to hardware information via their hardware survey, etc.
>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.