The age at which one is considered a legal adult in the US is typically 18. What defines a legal adult is ability to legally work, participate in contracts, vote, marry, give sexual consent, and join the military. Being a legal adult under 21 does not give one the right in most states to purchase alcohol, however.
The age one becomes a legal adult in the US is largely the age at which most children are considered adults in the world. A few countries set this age lower, but in most of the Western world, one is considered an adult at 18.
In some cases, a child under 18 is a legal adult especially for the purposes of marrying or signing contracts, prior to 18. This is rare and tends to occur if a child sues his or her parents for emancipation. An emancipated child is still a “minor,” however, and may not vote until he or she turns 18.
For many parents the term legal adult does not mean their children possess certain adult characteristics. For example, being an adult implies being able to make mature decisions, participate in civic matters, have self-control, and be responsible. Some 18-year-olds simply lack these characteristics and need further time to develop them. It concerns some parents that an 18-year-old can make life-altering decisions, like marrying early or joining the military, which may not have been made with significant forethought.
There is very little that parents can do to rectify such a situation. Many deplore that they no longer have any type of control over their young legal adult children. Ironically, for the child, perhaps, is that this type of control inherent in being an adult, essentially means a parent has no further legal responsibility to care for you.
Even if a newly legal adult is still in high school, a parent is under no legal pressure, frequently, to continue to provide housing or care. Some exceptions to this rule have been proven, particularly in cases of divorce where a spouse still claims child support, or help with payments for college. Some parents feel that continued financial support really attests to the fact that the child is really not quite an adult yet, regardless of age.
The issue of legal adulthood is a very sticky one for children in foster care. At 18, they essentially age out of the system and the state no longer has the burden of supporting them. In some cases, foster parents will continue to parent a child who is technically an adult. However, some children find themselves adrift at the point when they could most use the help and support of kind adults.