Having a child doesn't guarantee you have a dependent. The IRS has a string of tests for whether your child -- who may also be a stepchild, foster child, adopted child or grandchild -- is a "qualifying child." Even if you claim the exemption for years, you have to stop if your relationship eventually fails the IRS tests.
You have to stop claiming your child when she turns 19, unless she goes on to become a full-time student. In that case, you keep claiming the exemption until she's 24, as long as she meets the other qualifying-child tests. After a child reaches age 24, you can only claim her if she's completely and totally disabled. If she marries and starts filing a joint return, even if she's under 19, you lose the right to claim her on your taxes.
You can only claim a qualifying-child exemption if your child lives with you for at least half the year. The IRS offers a number of exceptions for this test. If your 20-year-old is at college eight months of the year, you can still claim him; if he stays with relatives while you're on military duty, he still qualifies. If you divorce and your son winds up living with your ex most of the year, however, you usually have to stop claiming him on your taxes.
You lose your dependent exemption if you don't provide at least half the child's financial support for the year. Support includes food and the fair value of her lodging -- what she'd pay for the room if she were renting -- as well as money spent on clothes, medical bills and school. If you pay $4,000 a year for your daughter but she earns $5,000 that she uses to help pay for college, you can't claim her. If she earns income but doesn't use it to support herself, you keep the exemption.
You have to stop claiming your son as a qualifying child if he moves out -- but you may be able to claim him as a "qualifying relative." The IRS rules are complex, but generally speaking, a qualifying relative can be someone who lives with you most of the year, or close relatives -- including children, parents, siblings -- who don't live with you but meet other tests. As of 2012, if your child has income of $3,700 or less and you provide more than half his support, you can claim him as a qualifying relative.
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