In at least Western history, it might be easier to find times when military service, particularly service in the ranks (broadly speaking) was not left to the poor and marginal. In ancient antiquity, some of the Greek states and the Roman Republic made military service a civic duty for the middling and the elite, but these systems collapsed under the strain of more frequent military campaigning over wider areas and the need to garrison captured territory.

In the medieval period, political and economic power was generally correlated with military service among the elite, and medieval peasants were not the unwilling spear-carriers that they're often made out to be. However, military service was still a dangerous and difficult way to make a living, and did not strongly appeal to those who had other options.

As you noted, this trend continued into the early modern period. In countries that did not institute conscription, armies were largely, though not exclusively, manned by the poor and desperate. There were exceptions to this, of course: Louis XIV appealing to his people to serve in his wars, and the rage militaire in British North America after Lexington and Concord. In at least the latter conflict, the patriotic rush to arms faded within a year or two, and most of American Revolutionaries' regular army was made up of people who were younger, poorer, and blacker than the average American.

For more on the Marian reforms that opened up enlistment in the Roman army, see Adrian Goldsworthy, Roman Warfare. For the Continental Army's separation from the population at large, see Charles Royster, A Revolutionary People at War.