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.