I know once a Roman was elected to the position of Quastor, Aedile, Praetor or even Consul he would be a senator for life.

Sort of. The Sullan lex Cornelia de XX quaestoribus increased the number of quaestors to twenty and provided that quaestors entered automatically into the senate. Traditionally the lectio senatus had been transferred by the lex Ovinia of 312 to the censors, who adjusted the senatorial roll during each lustrum. During Sulla's time that had become irregular, hence in part the provision for automatic entry for quaestors. While no earlier law appears to have provided that former magistrates take up a senatorial spot it was by the third century at the latest customary for magistrates to sit in the senate once their year was up, and every five years during the lustrum the censors tended to confirm these spots in their lectiones. During the period of regular censorial function technically speaking anybody could become a senator if enrolled by the censors, and a number of important Romans (e.g. Scipio) were enrolled "before their time" by the censors, without having held any senatorial magistracies at all.

The Sullan lex Cornelia de magistratibus formally codified the cursus honorum into law, providing that eligibility for curule magistracy depended on having held the proper offices in order, beginning with the quaestorship. Rare exceptions occasionally occurred (Pompey) but strictly speaking after Sulla since a man was not allowed to hold curule magistracy (excepting the curule aediles, who were nonetheless restricted by age and therefore fit naturally into the arrangement) without having held the quaestorship first all men who held such magistracies would have been senators already. The case of Lentulus Sura is a curious one, however. Lentulus had been cos. 71 but the censors of 70 ejected him from the senate. When he was executed in 63 as one of the Catilinarian conspirators he was a sitting praetor. Which means that he got reelected to the praetorship. Presumably he did not run again for the quaestorship, though we do not know that for sure, as he did not have to, having held it in 81. Sallust says nothing about the reasons for his second tenure as praetor, but Cassius Dio claims it was "in order to return to the senate" (ὅπως τὴν βουλείαν ἀναλάβᾐ). Formally speaking there doesn't seem to be much reason to think that Lentulus would have recovered his senate seat in 62, but since magistrates sat in the senate (without technically being senators) presumably Lentulus would have reentered by custom, if not by law.

The tribunes are a problem. The jurists and antiquarians recorded that people who sat in the senate were divided between senators and those who were allowed to give sententia to the senate. The texts of Republican laws, when we have them through epigraphic or other documentary evidence, support the phrasing. And a lex de repetundis from 123/2 defines magistrates with respect to their activity in the senate as men who have been in the senate or who were supposed to be in the senate, which indicates that formally speaking magistrates were no longer senators during their years. The problem is that from the earliest date the tribunes had sat in the senate, but did not conform with either of those categories. Traditionally the earliest tribunes had sat on their tribunician benches at the doors of the senate during meetings, unable to say anything but able to exercise their veto--"intercessio," lit. "walking between"--by physically preventing senators to leave the senate house. This apparently was still the normal procedure of the tribunician veto in the senate in Cicero's time. Now this doesn't present much of a problem in and of itself. The jurists and antiquarians are pretty unanimous in their opinion that the tribunes are not magistrates, as they are not elected by the entire people, but that over time they gained the recognition of various powers. That the tribunes don't fit into the normal structure is a given, and it is not surprising that at some point the tribunes gained the right not only to speak in the senate like everybody else but also to propose motions before the senate and even to propose motions contrary to what the magistrate who had convened the meeting of the senate had proposed. The problem is that a single source, Aulus Gellius, records the jurist Ateius Capito (Varro apparently seems to have agreed as well) as saying that the tribunes were not made senators until the lex Atinia but that they nonetheless had the right to convene the senate and propose motions to it even before the law.

The lex de repetundis of 123/2 lists tribunes as separate from the regular magistrates (i.e. those that it lists as having held or having been intended for senatorial seats), down there with the prefects and so forth. But we know from Appian that in 102 after Saturninus' tribunate the censor Metellus attempted to bar him from entry into the senate. Lintott therefore places the lex Atinia between 123/2 and 102 and posits that presumably it must have automatically qualified former tribunes for the senate. Gellius, and his authority Ateius Capito, surely are right here, because they refer directly to a law. But this law is only known from this single source, and we don't really know what it does. Did it permit former tribunes to be considered for entry into the senate? Did it treat them like the other magistrates, who were allowed to sit in the senate as if they were senators between the ends of their years and their formal entry during the next lustrum? We really don't know. It seems pretty hard to believe, in my opinion, that the lex Atinia automatically enrolled former tribunes into the senate when the quaestors did not automatically enter the senate until 81. Nor am I aware of any other examples besides Saturninus of men who we are told about being senators after their years as tribunes.

In fact, I'm not convinced by Lintott's very brief argument at all. Gellius' quotation of Capito says that "'the tribunes of the people,' he says, 'had the right to convene the senate, though they were not senators until the Atinian law'" ("Nam et tribunis," inquit, "plebis senatus habendi ius erat, quamquam senatores non essent ante Atinium plebiscitum"). Whatever the hell Gellius is talking about he must be talking about tribunes during their years, not afterwards. Senators did not have the right to convene the senate, so he must be talking about tribunes as having the rights of magistrates while not formally being magistrates, otherwise his argument is not parallel and makes no sense. Magistrates had this strange place as both senators and non-senators, and the tribunes would have been kept formally separate from this without some sort of law regularizing it. Whatever exactly Capito is talking about, I'm not convinced that it can be taken as an indication of anything about the tribunes after their years. Like I said before, I don't know of any tribunes who entered the senate after their years without having already been part of the senate. Lintott may well be wrong in dating the Atinian law to before 102, because he forgets Saturninus' career. Saturninus had been quaestor 105/4, and although quaestors did not automatically enter the ranks of the senate until 81 nonetheless Saturninus held the important Ostian quaestorship and came from the Saturninan branch of the Apuleian family. Saturninus' quaestorship was riddled with scandal, but there seems to be every reason to think based on the careers of other men that he would have continued to sit in the senate between 104 and his election to the tribunate. The next lustrum wasn't until 102, by which time Saturninus had held the quaestorship and the tribunate, a huge justification for his enrollment since at this period senators were enrolled from men who had rendered high services to the state--i.e., held elected positions. Appian does not say that Saturninus was supposed to enter the senate because he was a tribune, he says that Metellus almost barred him entry when his tribunate was over. The evidence for a lex Atinia that granted automatic entry into the senate for tribunes when higher-ranking magistrates were still not automatically enrolled is pretty thin to begin with, and it gets even thinner when we recognize that Saturninus' senatorial seat does not necessarily have anything at all to do with his tribunate. In 102 Saturninus was, by the conventional standards of Roman high society, eminently qualified to become a senator, and had every expectation of being enrolled, law or no law. That Metellus very nearly did not enroll him does not need to be connected to his tribunate, and if a law required him to be enrolled it really doesn't make any sense--there is no indication that censors after 81 could just decide that former quaestors would not enter the senate despite the law.