Medieval people grew up fast. We now think of the teen years as a stage between childhood and adulthood. For medieval people, in many ways adulthood arrived then.
As I noted in discussing medieval seven-year-olds, birthdays and exact ages were less important in the Middle Ages than they are now, but one was assumed to know right from wrong at age seven and be ready to take on a lot of adult responsibilities at age fourteen, with twenty-one being the age when one had normally learned all that there was to learn.
Age fourteen was considered a suitable marriage age, although, in practice, most people married later than that. It was also the official age of puberty, and in practice probably also the real age of puberty--as indeed it continued to be until a few generations ago. We now hit puberty around twelve or earlier, but this is because we, as a group, are better nourished than our medieval ancestors and are exposed to more hormones in water and food.
Teenage boys undergoing knighthood training would complete their training in their mid to late teens and be ready to ride off to war or tournament or to hassle the neighbors. Boys or girls who had been set into the monastery by their parents as children would decide around fourteen or fifteen if they wanted to continue in the cloister (as most did) or return to the secular world.
A boy who was going to go to the university (remember, girls couldn't go until the nineteenth century) would head off around age fourteen, rather than our age eighteen. He might get both his BA and his MA by age twenty-one and be ready to teach the undergraduates.
A town boy who had been apprenticed to a guild master to learn a craft (such as leather working, gold smithing, wagon making, and so much more) would make the transition in his early teens from being an apprentice to being a journeyman, someone considered skilled enough at his craft to actually be paid for his labor.
By the time a peasant boy or girl was a teenager, they were taking on full responsibility for farm tasks. This is still the case in many rural areas, where you will see fifteen-year-olds driving a tractor or a hay wagon.
There are numerous examples of medieval teenagers leading armies. Both Charlemagne and Louis the Pious had sons who led rebellions against them before the boys turned twenty. One of the most famous teenaged war-leaders was Joan of Arc, who first heard angel voices telling her what she needed to do when she was thirteen, who was leading men into victorious battle at seventeen, and was burned at the stake at age nineteen.
Joan of course was unusual by any definition, but really it was not considered odd before the twentieth century for teenagers to take on adult responsibilities. Look at Mozart at the end of the eighteenth century, composing as a small child, taking musical commissions when still extremely young by our standards.
Medieval teenagers were presumably just as potentially surly, obnoxious, or reckless as modern teenagers. But it was not a "stage" in which one was expected to be like that. (Even the term "teenager" is very modern.) Rather, these were traits that the real grownups wanted them to overcome as quickly as possible.