The Magna Carta has many points that refer to someone "under age". What does under age mean? Reference?

Answer 1

It's not defined in the Magna Carta, but since most of current English and American age laws state that you're an adult at 18, then my guess would be that the common understanding of being "of age" in 1215 was similar to what it is today.

So, "of age" is 18, "under age" is 17.

Answer 2

Current law defines adulthood at 18, but the "age of reason" is 12.

Answer 3

Underage means not an adult - as in

"The guardian of the land of an heir who is thus under age"

In other words, not able to assume the responsibilities of an adult.