Hi Kathie,
Robert was probably born in Pennsylvania, I am guessing that he may have migrated with his parents to South Carolina after 1760, but I have not yet found any details. He may well have been in some other state when (if) he enlisted, and Virginia doesn't seem completely impossible. I realize that it may be grasping at straws, but when I saw the (very brief) mention on this document, I thought it was worth asking...