When you emphasize the cruelty and meanness of the police, they shot a poor, unarmed, kid.
When you emphasize the bravery and dignity, his personal merit, then he's a man.
You use whichever makes your rhetorical task easier. Complicating this kind of rhetorical word choice is the utter inability of many sources to use the word "boy." I guess "minor". Too often that shows up as "underaged minor," which is unnecessarily redundant (can you have an "underaged adult"? An "adult minor"? Sure, there are emancipated minors, but that leads to a whole different terminological thicket.)
It was the same way during the Iraq War when some girl was, IIRC, raped, killed, and her body burned to try to hide the crime. She was a girl when sympathy was needed, as it usually was, but at times when her dignity and self-worth was important she was honorarily spotted 4 years and termed a "woman". In other cases, 14-year-old girls make really important life decisions--abortion being a notable example. In this case to call them "girls" is usually seen as an insult, a way of disempowering the "woman".
With Michael Brown there was a bit more spin. He was heading off, news reports said, to some "college." This was a good thing, and showed he was smart and working to overcome his situation through higher ed. While his friends were going off on sports scholarships, he was just going to attend college. Since I'd never heard of it I checked. It's one of those private "vocational/technical" institutes with dozens of local branches to train pepole in everything from being a dental assistant to a HVAC repairman. In other contexts these are unmeritorious scams out to delude people and provide no education; but when necessary, it's suddenly an important educational resource.
A lot of people make such choices unconsciously. Some make them consciously. Some spot the rhetorical devices used in audience manipulation.
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