I don’t keep track of that; I recall someone being ordained at an early age (21 sticks in my memory, which may not have much to do with the story) and as I recall, he was dying when ordained.
Tonsure was at one point a minor Order. There were others also, in a step fashion.
As to becoming a monk, that is going to depend on the order one chooses, as they likely have a minimum age. As to prior history, one needs to keep in mind that education was far different 100, 500, and 1,000 years ago and is essentially irrelevant except for “that was what happened then”. Likewise, neither now nor historically do I have any information.
Keep in mind also that “becoming a monk” may be a several year process; becoming a postulant or equivalent may be considered by some to “becoming a monk”, but is a bit more of a “come, let’s look at you and you look at us and see if we both wish to proceed further”.