There are actually a number of instances in American history, including in the early days of the republic, where legislation regulating "economic inactivity" (as the individual mandate is argued to do) was passed:
http://www.americanprogress.org/issues/2011/01/clearly_constitutional.htmlI mean, look at the first bullet point. George Washington signed a bill passed by a Congress made up of people who wrote and signed the constitution that required people to buy guns. If the individual mandate is unconstitutional, so is one of the very first laws, signed by our very first president, written by the people who wrote the constitution.