The Oxford English Dictionary traces singular they back to 1375, where it appears in the medieval romance William and the Werewolf. Except for the old-style language of that poem, its use of singular they to refer to an unnamed person seems very modern.
.....The well-respected Chicago Manual of Style still rejects singular they for formal writing, and just the other day a teacher told me that he still corrects students who use everyone … their in their papers.....
I have to differ; your reference to its first apparent usage in English literature is no doubt correct; but the actual use of it has been employed ever since in common language.