The United States Constitution has three requirements to become a Senator: be over the age of 30, be a U.S. citizen for at least nine years, and be an inhabitant of the state you seek to represent. Each state has two Senators, and each serves a six-year term.
The U.S. also has senators who work on the state level. In order to become a state senator of Texas, any candidate must be a U.S. citizen, be a resident of Texas for at least five years, a resident of the district for at least 12 months, and be over the age of 26. State senators serve four-year terms.