A 15 year old could be working in many places - probably at least has a functional understanding of value for money based on past experiences, a 5 year old almost certainly doesn't.
Which, unfortunately, doesn't mean anything. Someone can have a full time job at 15 on this planet, earn a living wage, have a bank account, and for all intents and purposes be financially independent. A five year old pretty universally cannot.
There is still a huge difference between 5 and 15. I am not saying the 15yo "got what they deserved" by any means, I'm just saying that it doesn't help anyone to pretend that they are developmentally identical. Laws may be descrete but morality is surely continuous.
A 15 year old likely makes some of their own financial decisions, occasionally, or even often, buys their own food, entertainment, etc. 15 year olds buy game consoles, computers and games all the time.
Because prosecutors sometimes care more about being seen as tough on crime and not compassionate towards young kids who do stupid things at an age where they're not blessed with an abundance of restraint and good decision making skills?
Because it's cheaper and more fair to set an arbitrary limit and apply it across the board than to try to evaluate each offender's maturity level individually.
Yeah my money, my decision. I had around $6k of bar mitzvah money and savings from working as a teaching assistant and summer jobs. I spent just over half on video games during high school and related costs. I made it all back the summer after high school as a tech intern and then spent it all going out drinking first year in university.
It's important because the article headline is a bit clickbaity. It's true that Facebook called children "whales" but not "as young as 5", since the linked chat transcript in the article mentions a 15-year old child.
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.
What is the relevant difference in this context?