The William Lightner Cowan letter state that David Cowan, the father of John, David and William of Pequea Creek, came to the colonies, ca. 1719/1720. This conformed nicely with the two "Davids" that are shown in the early Pequea tax rolls: one is obviously the David who died in 1758. None of the brothers were old enough themselves to have had sons old enough to be adults in 1724. So, the other David must be of the previous generation--the father David as stated by William Lightner Cowan.
I do not think it particularly unusual that Henry Cowan showed up first on the tax rolls. What is odd is the fact that John does not show up at all until his 1733 witness to the Fleming deed and his 1734 land warrant. He was the most "well-fixed," you might say of the 3 known brothers, so you would expect him there early on the tax rolls. Perhaps he lived in another area, and came in after the other brothers.
Henry Cowan may have been the eldest son; this seems to be the most likely possibility, but not the only possibility. its also possible that he was the brother to the elder David (Person:David Cowan (15), and thus uncle to the three brothers. The fact that he lived on, apparently, at least up until the late 1750s, however, suggests he was of the same generation as the known brothers. His close association with them suggests a sibling, rather than uncle/nephew relationship. The naming patterns of the known sons tend to support this:
There are a number of Cowans in our R1b1b2 DNA group who are very closely matched to the descendants of the 3 known brothers, and who absolutely CANNOT be descended from one of the 3 brothers. They being descended from this Henry (through his possible son George, who moved to NC) is the likeliest scenario.
For now, this is the best quess.
Terry--Tjohncowan 10:23, 9 February 2010 (EST)
Praphrase From: Mayhill, 1973:135
David Cowan and wife Hannah release to David Cowan....David Cowan died leaving will and bequests to three sons:Henry (eldest), David, and William equally upon paying share to his widow Mary, & to his other children; Since then Henry died being of full age, with will and beaquest to brothers David and WIlliam his third part, Oct 30, 1765