To claim your child as your dependent, your child must meet either the qualifying child test or the qualifying relative test:

To meet the qualifying child test, your child must be younger than you and either younger than 19 years old or be a “student” younger than 24 years old as of the end of the calendar year.
There’s no age limit if your child is “permanently and totally disabled” or meets the qualifying relative test.
In addition to meeting the qualifying child or qualifying relative test, your child must also meet all of the other tests for claiming a dependent:

1. Dependent taxpayer test
2. Citizen or resident test, and
3. Joint return test