The Child Tax Credit and the Additional Child Tax Credit are both computed on IRS form 8812 Additional Child Tax Credit.
As you answer the TurboTax interview questions, the proper calculations are made on the form.
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Look at your 2019 Form 1040 to see the child-related credits you received
Child Tax Credit line 13a
Credit for Other Dependents line 13a
Earned Income Credit line 18a
Additional Child Tax Credit line 18b
Child and Dependent Care Credit line 18d
You say 17 year old. If he had reached 17 by 12-31-19, he is no longer eligible for the child tax credit or additional child tax credit. What you get now is the (up to) $500 "Other dependent credit". There are no additional forms to fill out for that. If he qualifies as a dependent, TT automatically gives you the credit on line 13a of form 1040.