(ie the Oxford Classical Dictionary)

"...However, in or soon after 197[BCE], when the number of praetors was set at six, a new law stipulated that all consuls be ex-praetors. Henceforth, the basic progression was quaestor-praetor-consul. If the tribunate of the plebs and the aedileship were held, the former usually and the latter always followed the quaestorship; the censorship traditionally went to ex-consuls. The cursus acquired firther rigidity from the lex Villia annalis of 180 which set the minimum ages for each of the curule magistracies. L. Cornelius Sulla added an age requirement for the quaestorship, which he made cumpulsory. In the early Principate the pattern was extended. The vigintivirate became a prerequisite for the quaestorship; between these two offices it was customary to serve as a military tribune. All except patricians were obliged to hold either the tribunate of the plebs or the aedileship before reaching the praetorship. Career patterns beyond the praetorhship were less structured, though promotions to provincial governorships and the new non-magisterial posts show certain regularities. Established patterns of advancement eventually developed for equestrian careers, especially for the senior prefectures, but with greater variations than the Senatorial cursus."

-Oxford Classical Dictionary, Third Edition Revised.