The modern family may be made up of a wide variety of individuals, and it's not uncommon to take on the care of someone who isn't your direct descendant. The IRS allows you to claim nearly anyone as a dependent, as long as they meet certain criteria, and additional credits may be available if the child-adult relationship is qualified.
Qualifying Relationships - Dependent Child
You can claim someone as a dependent child for tax purposes if they are related to you in some other specific way. Stepchildren, natural, half- and step-siblings, foster children, or any of their children -- your nieces, nephews and grandchildren -- can be your dependents. You must provide at least half of their financial support for the year and they must live with you for at least half the year. Dependent children must be under age 19 if not in school, and under age 24 if enrolled in school full time, and they must be younger than you. Legally adopted children always qualify as your own children for tax purposes.
Qualifying Relationships - Dependent Relative
Other children may qualify as dependent relatives. These relationships get a dependent tax credit but may not qualify for other deductions like the child tax credit. If a child fits all the dependent child requirements but does not live with you, you may claim him as a dependent relative. If he is not related to you, but lives with you full time, you may also claim him as a dependent relative.
Residency Status Restriction
All dependents must have certain legal resident status, regardless of relationship. They must be U.S. citizens, legal residents or U.S. nationals. They may also be legal residents of Canada or Mexico. This means that many foreign student visitors do not qualify as dependents for tax purposes.
Neither you nor your child dependent can be claimed as a dependent by anyone else. A dependent child cannot file a joint tax return with a spouse, unless the return is filed only to claim a refund and neither partner has any tax obligation for the year.