Article I, Section 3 sets three requirements for senators: at least 30 years old, U.S. citizen for at least nine years, and resident of the state they represent when elected. The Framers debated extensively before settling on these numbers—delegates rejected citizenship requirements of 10, 13, and 14 years before approving nine years, making the Senate requirement two years longer than the House's seven. They set the Senate age at 30 (versus 25 for the House) three days after the House debate, passing it unanimously.
The Senate established in 1935 that senators-elect only need to meet age and citizenship qualifications when they take the oath of office, but must live in their state at the time of election.