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, starting on page 107:
Qualifying Child for the CTC
A child qualifies you for the CTC if the child meets all of the following conditions.
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 2020, 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 2020 if the child was born or died in 2020 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 Qualifying Child of More Than One Person in chapter 3.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.