The reigns of the 5 good Emperor's (Nerva 98-101, Trajan 101-117, Hadrian 117-138, Antoninus Pius 138-161 and Marcus Aurelius 161-180AD ignoring Marcus's co-emperor Lucius Verus) is conventionally marked as a high point or golden age for the Empire.

It marks a long era of political stability and economic prosperity for Rome with bloodless succession with few and entirely unsuccessful pretenders, there were two fairly serious Jewish rebellions under Trajan/Hadrian (the Kitos war and the Bar Kokhba revolt) but these are dealt with successfully again.

As for military affairs while there are fair number of wars (especially under Marcus Aurelius and Trajan) these all resolve in Rome's favour even if the Romans initially suffer defeat as with the early years of the Marcomannic wars seeing large acquisitions of territory (although some of it being abandoned very soon after), slaves and goods (Trajan's Dacian war purportedly flooded the market with gold so much as too seriously debase its value).

The earliest mention I have seen of it being called a golden age is Edward Gibbon in his book 'The History of the Decline and Fall of the Roman Empire' in the 18th century but I do think Roman historians like Cassius Dio were aware that ascension of Commodus was the end of particularly successful era in Roman history, I think its Dio said that Commodus marks a descent from a kingdom of gold to one of rust (or maybe its iron and rust).

Sorry for the shallow answer I have to go out.

Edit: Dio talks of Commodus being the end of a golden age at reference 72,36,4 in the book I have at least.