Final answer:
A guest in the U.S. must be at least 21 years old to consume alcohol with an ID, based on the National Minimum Drinking Age Act of 1984 backed by Mothers Against Dunk Driving (MADD).
Step-by-step explanation:
In the United States, a guest needs to be at least 21 years old to consume alcohol with an ID. This age requirement was established by the National Minimum Drinking Age Act of 1984 which mandated that states raise their minimum drinking age to twenty-one. Failure to comply with this federal law could result in a 10 percent reduction in federal highway funds for the non-compliant state. The act was heavily supported by Mothers Against Dunk Driving (MADD), which believed that increasing the drinking age would help reduce the number of dunk driving incidents, especially among the youth. After the law was passed, all states were in compliance by 1988. Despite the existence of this law, there has been ongoing discussion about the efficacy of the minimum drinking age in promoting responsible drinking habits.