In the United States, we generally consider minors who commit crimes to be a different class of criminal than people above 18. We do this because (AFAICT), there's a sort of societal agreement that wisdom/maturity is a logarithmic curve that begins to flatten in the late teens and 18 was picked as a legal threshold.
So if a 2 year old, 8 year old and 18 year old all shoot and kill someone, we prescribe much different levels of punishment based on their relative maturity. Sometimes, prosecutors decide to charge minors "as an adult" based on their behavior (Google for "X year old charged as adult" for examples). I assume that's what they're doing here.
FWIW, don't imagine that there was anything as elegant as "logarithmic curve analysis" used to decide that the age of majority is 18.
It's an age that was settled upon by common-sense consensus over a grand function of "Well, most Americans (descended from Europeans) thought it should be around 21," and that's probably because 21 is a nice, round number. Then the draft age got pushed to 18 because we needed more bodies for the meat-grinder in World War II, and the voting age followed around Vietnam when too many people asked "Wait, in what way is it just or fair we can force people to fight and die in a war who can't even vote?"
There isn't a lot of hard science (beyond the most ancient human science of all: observation across millions of data-points loosely confederated into "common sense") underpinning the age of majority.
It's true. Apparently, that's because mothers against drunk driving campaigned hard to have the drinking age re-raised to 21 after they got their hands on some evidence suggesting that it cut down on deaths due to car accidents.
As a society we generally make some allowance for a perpetrator's mental capacity. One aspect to that is we generally accept that teenage brains are not quite the same as adults.
