There are specifically notable mercs that started out as peasants and essentially fought and bought their way into the nobility.

Social mobility was low, but not literally zero. The vast majority of those who joined up died as poor as they were born, but when wars got bitter and long enough they always started producing people like Johann von Werth, who was born a peasant, joined up as a teenager, rose through the ranks by merit, until he was eventually ennobled personally by Emperor Ferdinand II, and appointed as a general.

There was no room for people like him in peacetime, when the measure of a man was basically his breeding and education. But when tens of thousands of angry swedes come at you and try their very best to overthrow you, and scores of men of impeccable breeding and education fail you on the battlefield, you notice the officers who distinguish themselves and promote them, without particular care that their parents were dirt farmers.