Medieval teenagers didn’t exist. No, really.
You were a child, and then you were an adult. This transition happened somewhere around 12-14, and by the end of it, you were an adult and expected to act like one.
The whole marrying at fourteen thing is overplayed these days — nobles and royals did sometimes, but ordinary people waited until they would be able to support a family, which put the average marriage age in the early twenties. But other adult responsibilities? They were yours now. Deal with it.
A few notable examples that highlight this:
When an underage king ascended to the throne, the minority council generally began the transfer of power when the king was about fourteen, and ideally handed over control when the king was about sixteen. That was the point at which he was considered to have been adult enough to rule.
There are records of fourteen-year olds being ordained as parish priests in times of scarcity such as post-plague — young, yes, but not that much younger than they would have been otherwise. Boys were usually turned over to that line of work at seven, the age of the beginning of formal education.
In free urban families — rather than unfree and rural ones, where you were going to take over your family’s land whether you liked it or not — boys entered into apprenticeships somewhere between ages eleven and fifteen, depending on the level of education their families gave them beforehand. They spent the next seven years or so being trained before slowly starting to strike out on their own. They were apprentices during their teenage years rather than independent craftsmen, but they were still working full days, usually longer than their masters’. This was the same age at which daughters went into domestic service at a wealthier home.
A wealthy childhood focused on education that was geared towards you taking on responsibilities for the family business as soon as possible, be that trade or ruling — or, if female, all the skills needed to run a household and be a good representative for your family. A poorer childhood was spent being trained in your father’s work, probably in the fields, or helping your mother in household work and whatever cottage industry she ran. Up until about seven, you were given more space to play, but once you were capable of helping it was expected that you would.
Once you could do the things you were being trained for, there you go, you’re an adult now. Off you go.
So, since most people in their teenage years were basically younger adults, what did they do for fun?
Well, what the other adults did, of course. Drink and gamble. If you were wealthy, hunt and play instruments and other things that took time and money.
Apprentices were treated by some writers in a similar way to how university students are treated now — adults, but reluctantly so. There are reports of gangs of apprentices roaming cities, picking fights that sometimes ended in deaths, and complaints from masters that their apprentices were sneaking out to get drunk and have sex when that was explicitly forbidden in their contracts. But remember, apprentices began in their early-to-mid teens. Most of these gangs were made up of what we would now call teenagers. So in that sense, they did much the same things for fun as the more rebellious teens do now. But they weren’t teenagers — they were adults.