Someone who wants to become a United States senator must be at least 30 years old. The designated minimum senator age was not fully debated by the 1787 Constitutional Congress delegates.
When debating the required U.S. senator age in 1787, the Constitutional Congress delegates looked towards England's parliamentary model. This model stated that members of parliament should be 21, and upon designating an age, the constitutional framers unanimously voted on making sure senators were at least 30, without having a debate on the matter.
In addition to deciding on a minimum age, the constitutional framers determined how long prospective senators should live in their chosen state before running for office. Unlike the English parliament, they decided not to implement strict citizenship laws. In addition, the minimum citizenship requirements varied between states. Eventually, they decided that a 9-year residency minimum was sufficient. Candidates must be a resident of the state they wish to represent at the time of starting their campaign.
Following the end of the Civil War, further restrictions were put in place. This included banning those who had taken an oath to support the Constitution, but later engaged in activities against the U.S. Initially, senator positions were not put to a public vote. This changed towards the beginning of the 20th century.