That everyone married as teenagers in the medieval period.
It turns out that teenage marriages were primarily in the upper class for dynastic reasons, and peasants mostly married in their mid-20s. The urban tradesmen class had the men marrying even later as no family would let their daughter marry a man who was not certain to have a trade to support her (and at some points the 'journeyman' phase of training was literally travelling hundreds of miles to learn from many masters).
It turns out that teenage marriages were primarily in the upper class for dynastic reasons, and peasants mostly married in their mid-20s. The urban tradesmen class had the men marrying even later as no family would let their daughter marry a man who was not certain to have a trade to support her (and at some points the 'journeyman' phase of training was literally travelling hundreds of miles to learn from many masters).