I've recently begun studying ancient Rome in my spare time. Something I've recently been looking at specifically was the infantry tactics round about the time of the second samnite war when they changed from the "phalanx with joints" to the newer model of fighting to cope with the mountainous terrain the Romans found themselves in.
I'm aware that centurions were expected to be the first into battle and the last out, and am curious about anything relating to that and the position itself..but primarily I'd like to understand the reputational gains this would have provided in ancient Rome and also how experienced a legionnaire would need to be to get it.
Thank you
This changed a lot over time, but I can try to sketch out some points, mainly relating to the earlier Republic.
When new legions were raised during the early Republic, the centurions of each line seem to have been elected by the soldiers serving in that line, starting with the most senior and then down to the junior (Polyb. 6.24). In existing legions, and once this archaic system of appointment had disappeared, centurions were appointed by their commander through his tribunes. This was probably based on experience, seniority and/or leadership ability (Polybius says ἀριστίνδην ("by merit"; the criterion is not clear); Livy 42.32.7 notes that during the 3rd Macedonian War, the principle of promoting based on seniority was violated and then restored, probably to make a moral point).
Once the organisation of a legion by age class into three groups was established, there were hence centurions of quite different social standing. The tenth (last) centurio of the hastati posteriores, so the second tier of the youngest and potentially worst equipped soldiers, was infinitely lower in social standing than the first centurio of the triarii priores, so the first tier of the best equipped and oldest (and even with the cohort system this hierarchy still operated on a cohort basis, it was just less granular).
Since early and mid-Republican commanders generally disbanded their armies after campaign completion, centurion rank was not a guaranteed career as such that could be maintained reliably. Note that during the Republic, officer ranks proper were restricted to the nobility, creating a firm barrier between them and enlisted men, whose careers were haphazard and could not go beyond first centurion. A soldier like Spurius Ligustinus in the 1st half of the 2nd century BC was hence not assured of anything and had to hope for advancement based on commanders' acceptance of his experience in previous campaigns during enlistment (Liv. 42.34). The outline of his "career" in the source also gives you some idea of the ages and distinctions required for promotion. As the army became more established, and then especially under the Empire, this changed. Becoming primus pilus, first centurion, for example, granted one significant monetary advantages upon discharge and raised one to the equestrian order, opening up a career in imperial (military or provincial) administration and ensuring significant, at least local social prestige. This might take a very long time to achieve (Juv. 14.197 gives the age of 60, but is satire).
Dobson, Brian, “The significance of the centurion and primipilaris in the Roman army and administration”, in: ANRW II.1 (1974), 392–434.
Sage, Michael M., The republican Roman army: a sourcebook, London 2008.
von Domaszewski, Alfred, s.v. "Centurio", in: RE III,2 (1899), 1962-1965.