A qualifying child can be any age if certain conditions are met.
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The dependent exemption allows you to reduce your taxable income by subtracting the exemption amount for every child for which you provide a certain amount of financial support. The dependent exemption is not an itemized deduction -- you can use it even if you don't itemize your deductions, and you can use it together with the standard deduction. You claim the exemption on Form 1040.
At the time of publication, you are allowed to deduct $3,700 from your taxable income for each qualifying child. If you have three qualifying children, for example, your exemption is worth $11,100. Add in a personal exemption for yourself, and the total comes to $14,800. This amount
is adjusted upward every tax year to account for inflation.
The Qualifying Child Test
The qualifying child test can be used to determine whether or not you can claim a child as an exemption. A qualifying child must be your son, daughter, stepchild, foster child, sibling, half-sibling, step-sibling or a descendent of any of these relatives, such as a nephew or a grandchild. Generally, a qualifying child must be under 19 at the end of the year. If he is a full-time student, however, he can be up to age 23. You can claim your child at any age if he is permanently and totally disabled. He must have lived more than half the year with you, and he must not have provided more than 50 percent of his own financial support.
The Qualifying Relative Test