1711 is the earliest possible year for the birth of Patrick’s and Margaret’s first child.
There are no apparent records of Patrick and Margaret having any daughters, but in all likelihood they did, and their first child could, in fact, have been a girl. Other people have promulgated the names of some daughters as though the names are fact, but I have not found any definitive proof that there were any daughters and, therefore I have not included any. Their sons were (in probable order by age) Patrick, Jr., James, Robert, John, Joseph, and Benjamin, with Jeremiah being another possible son. REF: Patrick, Jr., September 20, 1740; James, Robert & Benjamin, 1747; John, 1751; Joseph, June 25, 1753; and Jeremiah, April 4 - June 3, 1755 Although this would be considered an extremely large family today (especially since Patrick and Margaret almost certainly also had girls), it was not at all that unusual in eighteenth century Virginia—families averaged somewhere between six and sixteen children.(Albert Alan Rogers. Family Life in Eighteenth Century Virginia. Charlottesville, VA: University of Virginia, 1939. p. 255.)
Based on available facts, it can be deduced that either Patrick, Jr. or James was the oldest son, although it is most probable that Patrick, Jr. was the oldest for several reasons. First of all, under naming customs of the time, the first son was normally named after his father. (Allan Kulikoff. Tobacco and Slaves: The Development of Southern Culture in the Chesapeake 1680 – 1800. Chapel Hill, NC: The University of North Carolina Press, 1986. pp. 241 & 245 – 250.) Another fact to be considered is that Patrick, Jr. is the son who inherited the parcel of property where Patrick, Sr. lived. REF: September 6 & 7, 1770 By custom based on primogeniture the eldest son would inherit the homestead. (Louis A. Daniels. "A Little Something about Primogeniture," The Newsletter, Vol. 11, No. 10, April 1993. Prince William County Genealogical Society.) Patrick, Jr. was also named as executor of his father's estate after the senior Patrick's death,. REF: April 2, 1764 Also worthy of some consideration is the fact that Patrick, Jr. appears in documented records prior to any of the other sons. Based on these facts together, it is highly unlikely that Patrick, Jr. was not the oldest son.
The latest possible date for Patrick, Jr.'s birth is 1719. REF: September 20, 1740 comments. Discussions of the birth windows for the other sons can be found in their subsequent entries as noted above. This chart summarizes those dates: