Legally there is none. Both are considered minors, not capable of giving informed consent for (in many US states) marriage, employment, military service, voting, driving, or opening a bank account independently. And it's not like Facebook deserves rebuke for exploiting 5-year-olds and a Nobel Prize for exploiting 15-year-olds. But it's also true that the content of the article doesn't support the headline.

Edit: Even the math is wrong. If they're asserting that everyone's eligible to be called a "whale" who plays Angry Birds, and that the average age of an Angry Birds player is 5, then the minimum age ("as young as") of a whale isn't 5, it's less than 5.