If 18 is the age of emaciation in your state as it sounds like it is then "custody" is moot and the tax provision that allows a "non-custodial parent to claim the exemption no longer exists. *ONLY* the parent where the child lived more than half the year can claim a child under the age if 19 (or 24 is a full time student). The provisions that allow a non-custodial parent to claim ends at the age of emancipation per federal tax law.
**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.**
