Per IRS Instructions for Form 1040 and 1040-SR, on page 18:

Step 3: Does Your Qualifying Child Qualify You for the Child Tax Credit or Credit for Other Dependents?

3. Was the child under age 17 at the end of 2020?

No. You can claim the credit for other dependents for this child. Check the “credit for other dependents” box in column (4) of the Dependents section on page 1 of Form 1040 or 1040-SR for this person.

Additional Information

• IRS Publication 972 Child Tax Credit and Credit for Other Dependents

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