How old does my child have to be before I can allow her or him to be home alone? Under Maryland law, a child must be at least eight years old to be left alone in a house or car. State law also says a child must be at least 13 years old to baby-sit another child. Generally, it is left up to the parent to decide whether a child who is at least eight is mature enough to be home alone. Any time you leave your child alone, be sure the child knows what to do in case of an emergency. Baltimore County Department of Social Services' Child Protective Services (CPS) may become involved if a child of any age is left alone and is placed at risk of harm because he or she is unable to manage on his or her own.