In a recent lecture, a brilliant university professor of mine stated that the "18 = adult" convention is completely made up. She instead suggested that humans develop the ability to act as responsible, rational adults as their prefrontal cortex develops, starting at puberty and ending around the age of 25. When and why was 18 determined to be the age of "adulthood"?
Here's a short article. Basically, 21 was the traditional age of adulthood in the US, but drafting 18s for Vietnam created pressure for 18 as voting age.
This is something more suited for /r/AskAnthropology.