Child Tax Credit

On a 2020 tax return you lose the child tax credit in the year that your child turns 17   (not 18).    If you have a child who was over the age of 16 at the end of 2020 you do not get the CTC for that child.   Instead you get the $500 credit for other dependents for the child.  The rules for the CTC will be different for your 2021 tax return.

Look at your 2020 Form 1040 to see the child-related credits you received

PREVIEW 1040
https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing

Child Tax Credit line 19
Credit for Other Dependents line 19
Earned Income Credit line 27
Additional Child Tax Credit line 28
Child and Dependent Care Credit line 31 (from line 13 of Schedule 3)

https://ttlc.intuit.com/questions/1900923-what-is-the-child-tax-credit
https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-for-other-dependents

It depends, generally the Child Tax Credit is $2,000 for children under age 17. If your child is 17 and older by the end of the year then he/she might qualify for the $500 other dependent credit instead of the Child Tax Credit.

If both children are under 17 then please check line 28 for the additional child tax credit.

Please see What is the Child Tax Credit? for additional information.