Following is a partial answer provided by Bob's Genealogy Filing Cabinet, which has many interesting genealogy articles. The full article is here.
Actions Only Adults Could Perform
Under British common law, full majority was reached at the age of 21. Anyone under 21 was legally an infant. Only persons who had reached majority could perform certain legal actions:
- Buy or sell land without restriction
- Vote or hold public office
- Patent land
- Devise land in a will
- Execute a bond or promissory note
- Bring suit in one’s own name
- Be sued in one’s own name
- Serve on a jury
- Act as a guardian
- Marry without parental consent
Actions Minors Could Perform
Some legal actions did not require that a person be 21. For some legal actions, the law merely required that the person be judged capable of discretion. The age of 14 was generally accepted under common law as the age of discretion, and in rare individual cases (particularly females) it could be even lower. A minor could be judged by courts to be capable, just as an elderly person or an idiot could be judged to be incapable. Further, a father could give or withhold some or all of the rights of majority to a child, by “giving freedom”, though actually finding such a record is quite rare.
Generally speaking, children aged 14 and over could legitimately perform a variety of legal actions:
- Choose their guardian, or replace an existing guardian
- Apprentice themselves without parental consent
- Bequeath personal property (but not real property) in a will
- Witness deeds and contracts
- Testify in court
Boys aged 16 and over were obliged to serve in militias and could obligate themselves to military service without the consent of parents in most of the colonies.
Children aged 17 and over could act as an executor so long as other actions by adults were not required. (This is a relatively rare occurrence.)