For some reason there are all kinds of little bits of misinformation about the middle ages floating around out there. Most of them were started by Victorians and their romantic versions of history based on smatterings of data, but some were caused by badly researched fantasy or by things that have somehow unknowingly become tropes of the genre.
Here are some things that are not true.
People were shorter back then. Wrong. Archaeological evidence shows that people were just as tall at their full height. What may have led to this is that a poor diet may have caused people to take longer to grow to their full height, leaving many children to continue growing as late as 12 or 14.
People looked like they were super old at 30. Not sure how this one got started. Documentary and archaeological evidence shows us that people aged at generally the same rate (though perhaps a hard life of being a cotter could make you look a bit older earlier). Anyway, plenty of people lived to a ripe old age. The reason the "average age" is so low is that there were MANY stillbirths or child deaths, lowering the average. If you reached the age of five you were probably going to live for a fairly long time, but between 0-5 was a killer.
Knights wore plate armor. For MOST of the middle ages (600-1325ish) there was no plate armor. That means that chainmail was the most common form of armor throughout a huge period. Plate armor didn't begin to circulate again until the 14th century with the beginning of partial plate.
There was no change, development, or intellectual growth. That may seem true from the outside, especially since most of our sources from the early middle ages (the so-called "dark ages") are so thin on sources and later periods attack them for their ignorance. However, deep sea-changes in fashion, theology, technology and social formations happened continuously throughout the middle ages.
People never bathed. This is partially true. Noblefolk bathed more often than non-nobles, and the wealthy tended to wear pomanders filled with cloves and orange to mask their smell. However, never is a long time, and people bathed much more often than jokes and literature about the middle ages would have you believe.
The Nobility were a "class." Nobility was more like an adjective. Not until long after the middle ages in the pre-modern period did nobility become a closed class based on blood. People could "have nobility," and even "be gentle," but that did not preclude others from attaining access to this as well. Social mobility between the upper strata of freemen and the lower strata of nobility was actually fairly common, the barrier being at least semi-permeable.
Knights, chivalry, and courtly love ruled the day. Courtly love was an aspect of high medieval culture, and it has been greatly exaggerated in its importance. This is mostly the fault of C.S. Lewis, who loved courtly love. In fact, it may have just covered up affairs and the like, and either way was much less common than is generally supposed. In any event, the "practice" (hardly a set institution) was begun in the late high middle ages.
Peasants were all poor and benighted. Peasant is a word that signifies a free man of ignoble status. Peasants could be extremely wealthy, even landlords in their own right. The only thing "peasant" means it that they have no unfree (servile) duties to their own lords.
Fuedalism was the organization of society. Direct feudal links (where land was leased from the lord to his vassal) was less simple, sane, or useful than it is depicted. Many lords could owe vassalage to two or more other overlords. Most lords could keep their lands even against their overlord's will, and though kingdoms could disintegrate upon the death of their kings in early medieval society, as time went on various customs and appointed positions knit together the so-called "feudal state," giving it more cohesion than is generally assumed.