You lost the child tax credit for the older child, You lose the CTC in the year the child turns 17. Instead you get the credit for other dependents. You can still claim the 17 year old as a dependent.
https://ttlc.intuit.com/questions/1900923-what-is-the-child-tax-credit
https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre...
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