Generally, when you create an account online you’re told that if you’re under 18 you have to have a parent’s permission. So, what do most under 18 do? Well, they lie, of course, checking the box that says they have permission, or just putting in a date to make them “old enough”.
That’s why you have to give Nintendo props for at least trying to get more actual parental permission. Essentially, if you put in a birthdate that means you’re under 18 you’ll be given a prompt that a one-time, nonrefundable fee of 50 cents will be charged to a credit card. The purpose for this charge is to ensure that a parent or guardian has indeed given consent.
Of course, the system isn’t perfect. After all, as I mentioned earlier, they can just put in a date that would make them old enough, but there are bound to be at least a few kids who won’t think of putting in such a date and be forced to get their parents to come and cough up the 50 cents. But, at any rate, it’s a slightly more robust system than what many companies do, and it’s likely to net Nintendo a little bit of money to boot.