No. For a 2020 tax return you could get the child tax credit for a child who was YOUNGER than 17 at the end of 2020. If your child was 17 by the end of 2020 you could not get the CTC. For tax year 2021, if the child was younger than 18 at the end of 2021 you can get the CTC. But your child will also be too old to get it for 2021.
Amending your 2020 return will not change that. For a 17 year old you could only get the $500 credit for other dependents on your 2020 return.
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