You mean the child tax credit? You lose the child tax credit for the year in which the child turns 17--no exceptions. Instead you get the $500 credit for other dependents.
https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/child-tax-credit/L2lNhfGDl_US_e...
https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/additional-child-tax-credit/L6x...
https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-f...
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