What are some examples of real world adventurers? The ones which spring to my mind are:
- The vikings, who spread throughout Europe in late antiquity as, essentially, adventurers, mercenaries and pillagers, and who ended up founding kingdoms in Normandy, Sicily, the Middle East, and Russia
- The conquistadors who sailed across the Atlantic in search of gold, slaves and glory
- The explorers of Africa and Asia in the era of European empire building
- The forerunners of the settlers of the great North and South American interiors - prospectors, slavers, explorers, scientists
High medieval societies were rather hidebound by feudalism and religion. They were not the kind of societies which bred adventurers. Historically, adventurers have mostly been fostered by a mix of variables - relatively high poverty (the motivation to escape), relative lack of opportunity for advancement (another motivation to escape), relatively high change of glory (discoveries of new areas of the globe to explore), breakthroughs in travel technology (e.g. ocean going vessels that could make it all the way to China from Portugal) and relatively large numbers of young men with nothing to do (big pool to draw from). This was true, for instance, in post-reconquista Spain, when there were thousands of men who had successfully fought the Moors sitting around with nothing much to do except set sail for the New World. It was true in late 18th to late 19th Century Britain, when the population was booming and the interior of Africa was opening up, and there was a sudden opportunity to escape a life of misery working in some godforsaken shit hole factory. It was true in the mid-18th Century USA, when opportunity called thanks to the opening up of the West and, likewise, the population was booming and life in cities in the East was pretty crappy. I think it was also probably true in the Scandinavia of late antiquity - the Roman Empire had fallen and, hey presto, Europe was laying wide open for impoverished young men with nothing to lose to go and raid.
It makes more sense to me, then, to imagine D&D campaigns taking place in a context of societal change, when there are new areas of the map to explore thanks to discoveries or advances in travel technology (or perhaps due to catastrophe), and where people are seeing an opportunity to improve their lives and with relatively little to lose. It seems more realistic to set it in an era of technological or cultural revolution like the renaissance or the imperial age, then, rather than the Tolkien-esque quasi-medieval settings it has traditionally been found in. Out with long swords and chain mail, in other words, and in with sabres and muskets.