Roman senators typically served until death. Senators only lost the title if they were involved in a political scandal or infringed on the social codes of morality.
Every man who wanted to become a senator was required to complete the Cursus Honorum (course of offices). He had to be at least 25 years old and from a high-status family to begin working through these offices.
Although the senate had no term restrictions, the Consulship did. The senate elected two men each year to head the Roman Republic. They had to be a senatorial class, and would serve jointly for that year. This system aimed to discourage dictatorship, but the Consulship lost most of its power, as did the senate, when Rome became an empire under Augustus.