There you have it. The state of Virginia would be in violation of federal law if they tried to prohibit adults under 21 from purchasing a firearm. A state enacting their own laws that contradict federal law is a definite violation of the Supremacy Clause.
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Article VI, Paragraph 2 of the U.S. Constitution is commonly referred to as the Supremacy Clause. It establishes that the federal constitution, and federal law generally, take precedence over state laws, and even state constitutions. It prohibits states from interfering with the federal government's exercise of its constitutional powers, and from assuming any functions that are exclusively entrusted to the federal government. "