The Constitution sets three qualifications for service in the U.S. Senate: age (at least thirty years of age); U.S. citizenship (at least nine years); and residency in the state a senator represents at time of election. Here are the three qualifications: 1. They must be at least 25 years of age. 2. They must have been a citizen of the U.S. for the last 7 years. 3. Must be inhabitant in the district from which he or she is elected