In reading material for a course I'm doing on genealogy:
A minor was one under 21 but over 7. An infant was usually under 8, but sometimes referred to those under 21. However, in some countries an infant can mean they are aged under 1 year and a child someone who is over 1 but under 12 years of age.
Apparently in the 1750's an infant in the US was considered to be under 8 years but sometimes under 21 years but what about in 1850?