There's always more than can be said but I wrote an answer related to 18 you might find helpful. The answer contains a number of links to other questions on 18/21 that are useful.

The most relevant part from my answer:

Even with all of that sorting and adjusting in formal education, the concept of high school wouldn't really become the norm until the mid-1900's. The 8 years of grammar school, 4 years of high school, 4 years of college or university path was limited to a small portion of people on American soil - mostly boys, mostly white, mostly from families with means. Given they would grow up to become the men who set policy and laws, their experiences informed their policy making. So, in effect, they started school when they were 6 or 7, moved to a high school at 14 or 15, and left high school at 18 or 19. Hall and his friends pushed the message that it was deeply concerning if a child didn't move through education on the "normal" or standard schedule. This lead to a changes in policy at the local level that informed retention and advancement and a subsequent tightening down of age ranges inside different school buildings or sections of a school. By the time of the National Education Association's Committee of Ten report in 1894, which would inform the thinking and conversation around high school for several decades, the 8 + 4 + 4 structure was a given.

All of which is to say, it slowly became the norm that a young person would exit public education in May or June of their 17th year, turning or having just turned 18 at the start of the next phase of their life - which for most young Americans meant a job or family. Ergo, adulthood.