For the Child Tax Credit only, the child must be under the age of 17 on the last day of the year. The Child Tax Credit and the Personal Exemption for a dependent child are totally different. You can still claim the child as a dependent and receive the personal exemption for the child, $4,050 in 2016. The personal exemption is used to reduce your Adjusted Gross Income along with the standard or itemized deduction to arrive at your taxable income.
You should be able to claim the child as your dependent under the Qualifying Child rules if they meet all the requirements.
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.
