Adding to all the excellent answers here, there were also cases like that of Charles VI of France, who ascended the throne at the age of 11 in 1380. However, he was regulated to the position of a regent by his uncles who called most of the shots in how things were done. He had a very hard time, even after coming of age, asserting his position as the monarch. He eventually did, though that was also debilitated by the madness that seized him in later years.

Interesting question honestly. You got to wonder what was going through a noble's mind when he saw that the heir was young, sick, or mad. They respected traditions but they were also pragmatic enough to see that there were ways of working around an invalidated monarch.