If you read publication 501, it says that the special rules for children of divorced and separated parents are dissolved when the child is emancipated—which in most states is when the child turns 18. The child is not considered to be in the custody of either parent, the court order no longer applies, and no one claims the child as a dependent. The child files their own return and answers “no” to the question, can someone else claim you as a dependent this year.” See example 5 on page 13 here
https://www.irs.gov/pub/irs-pdf/p501.pdf
(In 2020, that will get the child an $1800 stimulus rebate even if they did not work. You will need to file an amended return to remove the child from your return and the child will have to file by mail since your prior claim will block their attempt to e-file. If the child has already filed a tax return, they will also have to file an amended return.)
