One of the common beliefs that armor was worn only by knights is probably due to the romantic notion of the “knight in shining armor”.
First, knights rarely fought alone, neither did medieval and Renaissance armies consist entirely of mounted knights. Although knights were the dominant force of most armies, they were supported and opposed by foot soldiers, such as archers, pikemen, and crossbowmen. During a campaign, a knight depended on a small host of retainers, squires, and attendants who lent armed support and looked after his horses, armor, and other equipment.

Second, it is wrong to assume that every nobleman was a knight. Knights were not born but created by other knights, feudal lords, or sometimes even priests. And, under certain conditions, people of non-noble birth could be knighted (although the knighting was often regarded as their admission into lower nobility). On some occasions, mercenaries or civilians fighting as ordinary soldiers could be knighted for exceptional displays of courage and bravery, while knighthood could be bought in later times.

In other words, it was not an exclusive right of the knight to wear and fight in  armor.