That's not an easy question to answer based on age alone or based on a child's emotional and mental development. And it's also not that simple when it comes to the Colorado Courts with regards to custody and child support. Our "Uniform Dissolution of Marriage Act" does not even define "child."
In a recent case (In Re Marriage of Tibbetts, 428 P.3d 686 (Colo. 2018)), Colorado determined that a person under the age of eighteen years old is still a child and the Court has jurisdiction to issue Orders for parenting time and decision-making. And therefore, at the age of eighteen the child can make his or her own decisions, and neither parent can force an eighteen year old to comply with the parenting time provisions. In contrast, the Court distinguished the age of emancipation regarding child support as it is an obligation of the parents and a child still has a right to be supported by parents until the age of 19, when child support ends.
Emancipation of a child for custody purposes ends at the age eighteen; emancipation occurs and child support terminates at the age nineteen.