Why is twenty-one considered the age of legal adulthood?

The practice grew out of British common law.

Before the Norman invasion, thirteen or fourteen was considered the age of adulthood, at least among the nobility.

But during battles, it was observed that thirteen- and fourteen-year-old nobles were not large or strong enough to carry the heavy armor and lance used in fighting.

The age was changed to nineteen and then raised to twenty-one, because nineteen-year-olds who inherited estates did not gain their property until two years later, owing to the lengthy legal processes involved.