Slate explains that the reason the legal age for Americans is 18 deals with the idea of voting. Stemming from the Vietnam War era, people did not agree that it was okay to send 18-year-olds to war without giving them the right to vote first. Prior to that era, the minimum voting age was 21. When the 26th Amendment was passed in 1971, the age moved down to 18.
It is speculated that allowing 18-year-olds the right to vote ties into the educational system, since many high school graduates are 18. Because obtaining a high school diploma was fairly universal across the country, those who were drafted into the Vietnam War were more educated and seemed to be able to take on the responsibilities associated with being an adult.
Those responsibilities include being able to vote as well as being able to seek medical advice without parental consent, sign contracts, live independently without guardians or parents, get married, make a will and sue on one's own behalf, according to The Florida Bar. An 18-year-old's other rights include being allowed to buy a car, earn credit and be selected for jury duty. Eighteen-year-olds are tried in a court as adults and are generally not claimed on their parents' tax returns. A male who is 18 years old must register with the Selective Service.