I was reading the question about the medieval central village being attacked, and someone was talking about medieval/early modern period justice. That got me to thinking, was one day considered in the medieval period and the next considered in the early modern periods? If so, when and why was this day chosen? Who defined it, and is it generally agreed upon or is it up for debate?
What actually is the definition of the two ages?
There are a lot of different dividing lines you could use - but I find one very useful one to be 1492. There were two important things that happened in 1492 that make it a good demarcation:
First, you have the Colombian voyages and first contact of Europeans to North America. Suddenly this makes the world a lot bigger and opens up the whole process of colonization that was probably the biggest hallmark of the modern period in a lot of different ways.
Secondly, you have the final elimination of Muslim settlements in Europe. The emirate of Granada fell in 1491 and in the following year a huge number of Muslims either emigrated or were enslaved, leading to the Christian dominated Europe you see since then.
It's important to remember that everything in history is on a spectrum, and even when you have what seem to be major "turning point" or epochal moments, the moments before and after them look very similar in lots of ways. So when you do a demarcation will depend on what you care about more - in art, you might say the middle ages ended with the Italian renaissance in the 14th century, whereas for some aspects of quality of life you might say you see the more modern turn in the 1600s. Place also matters: Russia and England look a lot more different in 1650 than they did in in 1100 in many ways.