There are 236 Ancestry trees including this George Farquhar. I've looked at a small number of them. They mainly give his birthdate as 13 April 1745, but they disagree on parent names, and none I've looked at gives a source reference for the birth I'm afraid. The most popular idea seems to be father William Farquhar 1710-1770 and mother Rebecca May 1720-1780, but without a proper source these are not at all solid. My guess is that a lot of the Ancestry trees are relying on each other
I also tried FamilySearch, but apart from the same marriage record as on Ancestry, he doesn't seem to be there.