Final answer:
A person under 18 years old with an unknown location can be considered a missing child if reported as such to law enforcement.
Step-by-step explanation:
A person younger than 18 years of age who has a temporary or permanent residence in a state, and whose location is not known, can be considered a missing child if someone reports them as missing to a law enforcement agency.
In such cases, efforts are made to locate the child and ensure their safety.