IRS Tests for Dependents
You might be able to claim an adult child who lives with you under IRS qualifying child rules. Alternatively, a child may be considered as a dependent adult. Some IRS tests apply to both categories. A dependent has to be a resident of Canada or Mexico if he isn't a United States citizen or resident. Dependents can't claim dependents or get a personal exemption on their tax returns. If your child is married, he can't file a joint income tax return unless it's just to collect a refund.
Qualifying Child over Age 18
There are two situations in which the IRS lets you claim an adult child as a dependent child. First, a child qualifies at any age if she is permanently disabled. Second, she qualifies if she is a full-time student until the year she turns 24. She must live with you more than six months out of the year, but if she's away at school that absence does not count as living apart from you. Finally, "child" in this context means not only a child by birth, but also children by adoption or marriage and siblings. Offspring of any of these are eligible. For example, nephews and grandchildren may count.
Children As Adult Dependents
The IRS doesn't have an age limit for adults to qualify as dependents, but there are income limits. An adult dependent must provide less than 50 percent of his income or support and can't make more than $3,950 per year as of 2014. As a relative, an adult child doesn't have to live with you. A non-relative would have to reside as a member of your household for the entire year. Finally, your child can't qualify as a dependent adult as long as she qualifies as a dependent child.
Tax Benefits of Adult Children
Any dependent gives you a dependent exemption. In 2014 this meant trimming $3,950 off your taxable income off the top. You might also qualify for the Child and Dependent Care Tax Credit if you have to pay a caregiver for your child so you and your spouse can work. You may be eligible to write off higher education expenses and student loan interest you pay on your child's behalf. You might also qualify for the Lifetime Learning Tax Credit or American Opportunity Tax Credit.