Wow, hang on. A man serving in the Legion could move all the way up to Primus Pilus (my spelling may be off) he was the most senior Centurian in the Legion and very important, but the post was still not that prestigious in the grand scheme of things. The Legate and the Military Tribunes were very much seperate from the rank and file, which is what Centurians were, the exception comes in the Praetorian Guard, where a Guardsman could be raised to Centurian rank in a regular Legion.

In the Roman system you very much had to take the long view. If you were an Auxilary your sons could be citizen Legionaries, if they got to Centurian then their sons might start out as Equestrians and become local Magistrates and their sons might make it into the Senate and up to the Consulship if they were lucky.

That said, if you were a citizen you could become Consul and history tells us it did happen, low men being raised up, but it wasn't common.