In order for anyone to claim a dependent on his/her tax return the following Qualifying Child Rules have to be met:
To be a Qualifying Child -
1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you unless disabled.
NOTE: However, if a divorce decree or court order gives your ex-spouse the right to claim the children, he can do so if he attaches key pages of that document to his tax return. Whereas, if no divorce or separation decree states that the noncustodial parent may claim the exemption or there is no written declaration from the custodial parent, tiebreaker rules are in effect. The parent who the child spends the most time with may claim the dependent.
more information about Qualifying Child: https://www.irs.gov/newsroom/a-qualifying-child
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