In book five of Livy’s history of Rome he says that Publius Licinius Calvus, the first plebeian to become a consular tribune, was “an elderly member of the senate.” I was under the impression that at this point in Roman history only patricians could be appointed to the senate but apparently that’s incorrect. Livy covers the Conflict of the Orders in detail but never (as far as I remember) mentions plebeians gaining access to the Senate, which seems like something that would be worth detailing. At what point were plebeians allowed to serve as senators?
Livy didn't know when plebeians were admitted into the senate and neither do we. Livy's not even sure how senators were enrolled in the fifth century and early fourth centuries. The tradition on the earliest consuls and senators was highly conflicting and extremely bare. Traditionally only patricians had been able to hold the consulship, for example, but plebeian families appear from the very first consulship--Brutus is a plebeian name. Certain features of the senate, such as the fact that only patrician senators chose interreges, suggests a patrician origin for the order, but at the same time there's no direct testimony and many of these features are religious in form, and therefore their patrician character is not surprising. It's a matter of great debate when plebeians were admitted into the senate or whether plebeians had ever been barred from it.