My Family Tree

Commonly believed David is William's brother. Assuming he was at least 21 years old in these records:
Martha Hurt married David Thomas [_If_ this is him!] in Amelia County, Va. by bond dated November 27, 1784 => b.<1763
Thomas, Ephraim and Caty Teal, dau. Adam and Mary,Jan. 19, 1791. Sur. David Thomas => b.<1770
1799 Franklin Co, VA Tax List: William, David, and Ephriam Thomas => b.<1778
In 1820 Smith Co, TN , he is 45+ years old => b.<1775
David is believed to have died in the early 1820's in Jackson Co, TN like brother William.

Note if born after 1770 David could be William's son instead - the eldest brother of Ephraim. If Ephraim followed English naming conventions, then David as 4th son would be his eldest brother.