Adults haven't been emancipated in this country since 1865.  Minors can be emancipated by court order or operation of state law, but that doesn't apply to an 18-year-old.  It's an interesting question, how Form 8332 rules apply to adult children.  If the court decree requires child support to continue past age 18, then the noncustodial parent can still claim the exemption.  (Courts often order payments to continue through high school graduation, which could be age 19.)  

Tell us the facts and circumstances, and most importantly, are both parents trying to claim the adult child?