Senators were former magistrates, that's it. Following an election to the quaestorship one would be enrolled into the senate, an advisory body with few official powers and which essentially was composed of former magistrates advising current ones. The minimum age for the quaestorship was 30, and senators continued sitting in the senate (except for when they were serving as magistrates--technically speaking one left the senate during the year one held magistracy) for life. Therefore it's sort of hard to determine an "average age"--we can establish a minimum, but senators continued on for life