The age of majority is when a youth is considered to be independent of their parents. For example, a minor cannot enter into a legal contract because they are not considered independent persons under the law.
Traditionally the age of majority has always been 21 for males and 18 or 21 for females, depending on the state. A female always gains majority on marrying, thus oddly you could have a situation where the husband is older than his wife, yet is a minor, but his wife is an adult.
The shift from 21 to 18 as the age of majority happened around 1970. For example, in Massachusetts it occurred in 1974.
In 1900 voting age was 21 in nearly all states.
The several United States generally followed the practice established under English Common Law of setting the age of majority at 21. To quote:
By the common law the age of majority is fixed at twenty-one years for
both sexes, and, in the absence of any statute to the contrary, every
person under that age, whether male or female, is an infant.
-- The American and English Encyclopedia of Law, Garland and McGeehee, 1900
The same source notes that by statute 17 states had the female age of majority set at 18. Those states were:
Arkansas, California, Colorado, Dakota, Idaho, Illinois, Iowa, Kansas, Maryland, Minnesota, Missouri, Nebraska, Nevada, Ohio, Oregon, Vermont and Washington.
Note that under ancient Saxon law the "age of discretion" was 14 for a male and 12 for a female and this prevailed for a long time in English Common Law, but this is only the age at which they may marry and become liable to committing felonies. Under English Common Law, to enter into a contract and be recognized fully as an adult you must be 21 and this was always the case. Here is a quote:
By the common law, every person is, technically, an infant, until he
is twenty-one years old; and, in legal presumption, is not of
sufficient discretion to contract an obligation at an earlier age.
The ultimate source which is authoritative is Institutes of the Lawes of England by Coke (1628-1644). The laws on infants are at 171b.