[Jamestown1630]

Well, argue it with hundreds of years of English usage.

[Chickensoup]

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.....