Nobody gets the additional child tax credit for an 18 year old. You lose the CTC when they turn 17. All you get for claiming a child over the age of 16 is the $500 credit for other dependents.
CREDIT FOR OTHER DEPENDENTS
https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre...
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