Age of majority

The age of majority is the age when a person becomes an adult by law. This means that they are legally in control over their own actions and decisions, and their parents are no longer responsible for them. When used this way, the word "majority" means having the full number of years to be an adult. The opposite is "minority", which means being a minor or child. The law in a given place may never actually use the words "age of majority" when deciding when people become adults. The age of majority is a legally fixed age and idea of adulthood which is different in different places. It may not match the actual maturity of a person's body or mind. The age of majority is 18 in the vast majority of jurisdictions, but ages as low as 15 and as high as 21 exist in some. 
Overview.
Once a person reaches the age of majority, there are some things they can do that they could not do before. These may include buying stocks and marrying without having to ask for permission. The ages that these things can be done are different depending on where the person lives.
Even after a person reaches the age of majority, there may be other age-based rules that they still have to follow, such as the right to stand for office in elections or become a judge. For example, the youngest a person is allowed to purchase alcohol is 21 in all U.S. states even though the age of majority is 18 in most states. The age of majority in the Republic of Ireland is 17, but a person must be over 21 years old to stand for election.
"Emancipation" is when a child is freed from the responsibility and care of their parents or legal guardians before they reach the age of majority.
In almost all places, minors who are married are automatically emancipated. Some places also do the same for minors who are in the armed forces or who have a certain degree or diploma.
Countries and subdivisions.
This is a list of the age of majority in various countries (or administrative divisions):
Age 15
Age 16
Age 17
Age 19
Age 20
Age 21