Your child must be under 17 at the end of the tax year in order to qualify for the Child Tax Credit.

Per IRS Publication 17 Your Federal Income Tax, Page 212:

Qualifying Child for the CTC

A child qualifies you for the CTC if the child meets all of the following conditions:

1. The child is your son, daughter, stepchild, foster child, brother, sister, stepbrother, stepsister, half brother, half sister, or a descendant of any of them (for example, your grandchild, niece, or nephew).
2. The child was under age 17 at the end of 2019.
3. The child didn’t provide over half of his or her own support for 2019.
4. The child lived with you for more than half of 2019 (see Exceptions to time lived with you, later).
5. The child is claimed as a dependent on your return. See chapter 3 for more information about claiming someone as a dependent.
6. The child does not file a joint return for the year (or files it only to claim a refund of withheld income tax or estimated tax paid). 
7. The child was a U.S. citizen, U.S. national, or U.S. resident alien. For more information, see Pub. 519, U.S. Tax Guide for Aliens. If the child was adopted, see Adopted child, later.

Adopted child. An adopted child is always treated as your own child. An adopted child includes a child lawfully placed with you for legal adoption. 

If you are a U.S. citizen or U.S. national and your adopted child lived with you all year as a member of your household in 2019, that child meets condition (7), earlier, to be a qualifying child for the child tax credit (or condition (3), later, to be a qualifying person for the ODC).

Exceptions to time lived with you. A child is considered to have lived with you for more than half of 2019 if the child was born or died in 2019 and your home was this child's home for more than half the time he or she was alive. Temporary absences by you or the child for special circumstances, such as school, vacation, business, medical care, military service, or detention in a juvenile facility, count as time the child lived with you.

There are also exceptions for kidnapped children and children of divorced or separated parents. For details, see Residency Test, in chapter 3.

Qualifying child of more than one person. A special rule applies if your qualifying child is the qualifying child of more than one person. For details, see Special Rule for Qualifying Child of More Than One Person, in chapter 3.