Criteria for qualifying as a dependent child
Child is taxpayer’s 1) descendant, 2) sibling, or 3) descendant of taxpayer’s sibling
Age:
Younger than taxpayer and either 1) under 19 or 2) A full-time student under 24
Age requirement does not apply if permanently or totally disabled
Child lives with taxpayer for more than half of the year
Child does not provide more than half of own support
Child is not filing a joint return
A U.S. citizen, resident, or national for any part of the year
Please see https://apps.irs.gov/app/vita/content/globalmedia/overview_of_the_rules_for_claiming_4012.pdf for more detail.