The age of majority is the age at which an individual can sign contracts (i.e., no more "defense of infancy"). The age of majority for signing contracts is 18 in most states, except Alabama and Nebraska, where it is 19, and Indiana, Mississippi, New York and Puerto Rico, where it is 21. For child support purposes, the age of majority is 18 in most states, 19 in Alabama, Colorado, Maryland and Nebraska, and 21 in D.C., Indiana, Mississippi, and New York, with exceptions for a later age of majority if the child is still in secondary school. As I have stated before the effective age of majority in NY varies it is less for criminal law for example.
http://public.leginfo.state.ny.us/menugetf.cgi?COMMONQUERY=LAWS
- Penal
Article 30 - DEFENSE OF INFANCY
§ 30.00 Infancy.
1. Except as provided in subdivision two of this section, a person
less than sixteen years old is not criminally responsible for conduct.
2. A person thirteen, fourteen or fifteen years of age is criminally
responsible for acts constituting murder in the second degree as defined
in subdivisions one and two of section 125.25 and in subdivision three
of such section provided that the underlying crime for the murder charge
is one for which such person is criminally responsible; and a person
fourteen or fifteen years of age is criminally responsible for acts
constituting the crimes defined in section 135.25 (kidnapping in the
first degree); 150.20 (arson in the first degree); subdivisions one and
two of section 120.10 (assault in the first degree); 125.20
(manslaughter in the first degree); subdivisions one and two of section
130.35 (rape in the first degree); subdivisions one and two of section
130.50 (criminal sexual act in the first degree); 130.70 (aggravated
sexual abuse in the first degree); 140.30 (burglary in the first
degree); subdivision one of section 140.25 (burglary in the second
degree); 150.15 (arson in the second degree); 160.15 (robbery in the
first degree); subdivision two of section 160.10 (robbery in the second
degree) of this chapter; subdivision four of section 265.02 of this
chapter, where such firearm is possessed on school grounds, as that
phrase is defined in subdivision fourteen of section 220.00 of this
chapter; or section 265.03 of this chapter, where such machine gun or
such firearm is possessed on school grounds, as that phrase is defined
in subdivision fourteen of section 220.00 of this chapter; or defined in
this chapter as an attempt to commit murder in the second degree or
kidnapping in the first degree.
3. In any prosecution for an offense, lack of criminal responsibility
by reason of infancy, as defined in this section, is a defense.
And specifically from
§ 2-1.10 Provisions relating to infants and minors
(a) Unless the creator expressly provides to the contrary, in any
instrument executed prior to September first, nineteen hundred
seventy-four,
the words "minor", "minority", "infant", "infancy",
"majority", "adult" and words of like import shall mean or refer to a
person or a class of persons under the age of twenty-one years or who
shall have reached such age, according to the context, and, unless
otherwise expressly provided in any instrument executed on or after
September first, nineteen hundred seventy-four shall mean or refer to a
person or a class of persons under the age of eighteen years or who
shall have reached such age, according to the context, except that any
designation of a testamentary guardian of a "minor" or an "infant" shall
refer to a guardianship of a person who has not reached the age of
eighteen years, regardless of the date of the instrument containing the
designation.
(b) This act shall not apply to distributions made subsequent to
September first, nineteen hundred seventy-four and prior to the
effective date of this act.
Family Court Act
§ 580-101. Definitions. In this article:
(1) "Child" means an individual, whether over or under the age of
majority, who is or is alleged to be owed a duty of support by the
individual's parent or who is or is alleged to be the beneficiary of a
support order directed to the parent.
(2) "Child support order" means a support order for a child, including
a child who has attained the age of majority under the law of the
issuing state.
(3) "Duty of support" means an obligation imposed or imposable by law
to provide support for a child, spouse, or former spouse, including an
unsatisfied obligation to provide support**************
The exact age is not included as it varies under the applicible code but is effectively 21.