Thats only in the western world. Originally before WW2 the of majority in most parts of the world used to be 21 as you where not considered mature until they reached their 20s. But in WW2 they lowered it to 18 so they could drag younger people into war.
Tbf the age of majority should be raised to 20. 18 and 19 year olds are still teenagers who may still be in highschool. And lets face it, lowering the age of majority to drag younger people into war is very despicable.
Ive talked about this subject with several conservatives (many which where either paleoconservatives or traditonalist baby boomers) a while ago, but whenever I proposed raising the AOM they called me a RINO and said that I was going "against the system" and that it "went against traditional values". Gosh I hate those people.