The question could be answered by comparison of the King James Bible (1611 version) with the Wycliffe Bible (1382) but would require some considerable research.
Here are two examples I found myself, which indicate that 'they/their/them' was not used singularly prior to the 16th century:
2 Kings 14:12
1611 every man to their tents
1382 ech man in to his ...
Yes, from the OED blog:
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. Here’s the Middle English version: ‘Hastely hiȝed eche . . . þei neyȝþed so ...
I'm afraid I can't give you a peer-reviewed answer. However, the use of the singular "they" by Shakespeare is well documented.
NYUlocal's discussion of the Swedish "hen", also claims the singular "they" was used in The Canterbury Tales, Hamlet, Mansfield Park, and The Universal Declaration of Human Rights.
More to the point, in &...