Are you asking about the child tax credit on your 2020 tax return---or the advance monthly payments for 2021?
The law for 2020 says that you get the CTC on your 2020 tax return for a child who is YOUNGER than 17 at the end of 2020.
For the new 2021 child tax credit the child has to be YOUNGER than 18 at the end of 2021.
The controlling factor is the child's age on December 31 of the tax year.
For the ordinary child tax credit (up to $2000, for years 2020 and earlier) the child must be "under 17" (age 16 or less) on December 31.
For the enhanced child tax credit ($3000, for tax year 2021), the child must be "under 18" on December 31, 2021.
If this seems unfair, (such as, your child's birthday is December 29 and they were of qualifying age for almost all the year but don't get the money), remember that the child also got the full credit in the year they were born, even though they were only alive a couple of days.