Marcus Brutus wrote: I have no idea what you are talking about Trojan. I do know that both Adrian Goldsworthy in his book on the Roman army mentions 25 year terms for legionnaires in the period under discussion. The Wikipedia page on Legionary confirms 25 year terms. Do you have original source material that disputes this?
From my interactions with you, it has become quite clear you gleam your knowledge about the Roman army from modern sources. That can be precarious road to travel. When reading Adrian Goldsworthy, he should provide a footnote or endnote citing the ancient source (Tacitus, Paterculus etc). If a historian does not provide such information, you can be assured it will be some theory developed academics.
In 13 BC, according to Isidore of Seville (Etymologica 9 34), Augustus decreed all legionaries must serve for 16 years and all Praetorians for 12 years (Cassius Dio (54 25 6), Tacitus (Annals 1 17-19 & 26), ILS 2649). In 5 AD, according to Tacitus (The Annals 2 80), Augustus lengthened the term of service of the legionaries to 20 years. Cassius Dio (55 23 1); (57 6); Res Gestae 3 17; Tacitus (Annals 1 78)
A papyrus concerning some Egyptian veterans of Legion X Fretensis being discharged mentions the soldiers "conducted ourselves over twenty [years] in ever respect as good soldiers should".
The inscriptions from Virunum in Noricum (EDCS-22300520), belonging to cohors Montanorum prima, records the following:
CIL iii 4846 Ti. Iulius Condolli f. Capatius 30 years service
4847 Ti. Iulius Adesdi f. Taulus 36 years service
4844 Ti. Iulius Adgelei f. Buccio 40 years service
11554 Ti. Iulius Giamilli f. Sextus 40 years service
The 16 years' service and then 20 years' service for the principate are divisible by batches of four men, so the system Polybius is describing appears to be still in use.
When civil war erupted in 69 AD, Tacitus (The Histories 82) writes that the first business of war was to hold levies and to recall the veterans to the colours. After a legionary has served his military term of service (16 years or 20 years), when discharged from the army, a legionary was still required to remain under the standards (eligible for call up). The writings of Cassius Dio, Paterculus and Tacitus provide references to veteran soldiers and veteran cohorts accompanying the army or acting independently of the army. (Cassius Dio (55 24 8), Paterculus (2 110), Tacitus (The Histories 4 21). In 69 AD, during the Roman civil war, Tacitus (The Histories 83) has Mucianus marching with the 6th legion and 13,000 veterans. Tacitus (Annals 3 21) gives a detachment of veterans (vexilla veteranorum) at "about" 500 men. For the campaign against the mutineers in Pannonia in 6 AD, 10,000 veterans and 10 legions (48,000 men), were levied. Paterculus (2 113), Tacitus (The Histories 2 18) mentions 1,000 reservists.
Therefore, the common citation of 25 years' service may thus be a summation of 20 years active service plus 5 years in the reserve (veteran status). So, if a historian claims 25 years' service, that historian should explain it as 20 years' required service and 5 years as a reservist, which means although discharged, he can be called up again when need during that five-year period as a reservist.
There is no ancient source that states a legionary must serve 25 years. There is also evidence that the legionaries during the period of the principate were also still being conscripted.