On 6 Jun 1781, James King witnessed a deed by James Head and his wife Sarah Head to Francis King. Francis King was likely the father of James King. Assuming that James King was at least 21 years of age in order to serve as a witness, he was born before Jun 1760.

The 1810 U.S. Federal Census of Spotsylvania County, Virginia enumerates the household of James King, age over 44 years. Also living in the household were a female, age 26-44, and a male age 0-9 years. Presumably, the female was Susannah King, the wife of James King. Assuming that James King and his wife were of similar ages, James King was not much older than 44 years. That James King was more than 44 years of age in 1810 implies that he was born before 1766.

1810 U.S. Federal Census
James King Household
Spotsylvania County, Virginia
James King, a: 45+
+Female, a: 26-44
Male, a: 0-9