Bingley points out: use of they/them/their/theirs to refer to an unspecified singular person antedates the political correctness movement by six hundred years.

Interesting link, Bingley. I have added it to my English Language bookmarks.

I had thought that the use of the second person plural for singular had predated the corresponding use of the third person plural by a couple of hundred years. I'll stand by my notion even more firmly now that the modern objection to this usage has *its roots in PCphobia. If this helps you to avoid tripping over me in the dark, then so much the better for both of us.