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.
