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It seems that the best indicator I have of David's age is the age of one of his sons. David's son, William Drennon married Elizabeth David 8-8-1777, Lancaster County, Pennsylvania, at the Presbyterian church. In addition, grandsons,David and William Christie, were mentioned in the will.Said grandsons were the age to join the regular army in 1791. Therefore, when the will was written David was a grand-father.
The children of David Drennon were: Thomas, John, William, Agnes, and David Jr.
Executors of the will were Thomas and William Drennon.
Perhaps, one could estimate the birthyear of David as 1737.