@lcmcap
From the day your child turned 18, the court order regarding dependent arrangements on your federal tax return is no longer enforceable. from that point, the child is not considered to live with either parent, and neither parent can claim the child as a “qualifying child“ dependent, because that status requires residency.
The other kind of dependent is “qualifying relative.“ For a parent to claim the child as a qualifying relative dependent, the parent must pay more than half the child’s total living expenses and the child cannot have more than $4300 of their own taxable income. It doesn’t matter what the court order says, and it doesn’t matter where the child actually lives. It only matters who pays more than half of the child expenses.
If the child earned more than $4300 in the year that we are talking about, the neither you nor the other parent can claim the child as a dependent. You could file an amended tax return, or wait and see if the IRS catches up to you. The IRS has three years to correct your return from the date you filed it. The child would also be eligible to amend their tax return to uncheck the box that says “I can be claimed as a dependent by someone else.” This may qualify them for additional credits, but we can’t know without a lot more information. If the child earned less than $4300 that year, then you have to do a calculation to see who paid the most support. Support includes tuition, room and board, travel, medical expenses, clothing, and entertainment. Support that the child paid for them self includes their own earnings they spent on their own living expenses plus student loans taken out in their own name, because they must be repaid. Whichever parent paid more than half can claim the child as a dependent, and that might even be your ex. If no one paid more than half, for example if your ex paid 40% and you paid 40% and the child paid 20% of their own expenses, then no one claims the child as a dependent.
