…but why aren’t people 21 and over “majors”?
When a person reaches the magical age, they are said to have “reached majority”.
Wiki article. So, in a way, they are “majors”.
ETA: The age is different in different places; not always 21.
CookingWithGas, the age of majority is 18 nearly everywhere.
Not for drinking, though.
KneadToKnow, in most countries the age of majority is the same for a lot of things for which it’s different in the USA. Including for drinking, for voting, for driving cars, and for being elected to office.
The youngest senator in Spain wasn’t able to vote for himself the first time he got elected - you have to be 18 to be sworn in and 18 to vote, but the law doesn’t say you have to be 18 to run. The election took place 2 days before his 18th birthday 
Oh, I have no doubt. But since the OP, the person I responded to, and I are all in the US, and no one had yet mentioned foreign countries, I didn’t feel the need to open that door.
Ah, I didn’t know that “everywhere” referred only to the US 
The minor/major distinction is found in a number of places.
Constellations have names like Canis Major and Canis Minor.
You major in a subject in college, but can also take a minor with fewer courses in another subject.
The real difference is that minor, or lesser, is normally used just to distinguish a similar example from what is considered to be the “true” example. John Smith, Jr. is distinguished from his father John Smith, Sr. by being known as Junior when the father is seldom considered to be Senior. Minors in law are distinguished from the norm, which are people of age.
This is standard usage in English. Similar usages include feminine endings like -trix or -ette or -ess.
Yes, this means that adult males are considered to be the norm against which every variation is measured, and this norm has only slowly been changing. So what else is new?
You guys are off topic. It’s a language question. Why do we call underage people “minors” but never call adults “majors”? Yes, I know that they say “legal majority” but nobody ever says they only sell alcohol to majors.
Once again, because adults are the norm. The norm doesn’t need a special name. Only deviations from the norm require special names.
We don’t seem to mind saying “adults.”
Right. On the one hand you have adults.
On the other you have minors, children, teens, teenagers, adolescents, youngsters, kids, school-age, and juniors. If you go down in age, you run into a hundred more words, like toddler, infant, baby, tween, tad, sprite, and on and on.
That’s the way the English language works. There’s no need to have a balance of terms in everyday language.
They exist:
but they aren’t necessary and they are seldom used.
So why use major? Adults fills the niche solidly. You can vary it with grownups if you want. English speakers voted with their tongues. The same answer applies to most questions about English.
Because language doesn’t work in a strictly logical fashion.
Exapno has probably given you as logical a rationale as you’re going to get.
I wish English had a more chalant attitude.