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If you are talking about the child tax credit then if one child is 17 or older then something has changed - the child must be under age 17 for the credit.   

Additionally, If a child has turned 18, and is an adult according to the laws of your state, then only the parent that the child lived with can claim.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**