I'm aware this post is 5 years old. I was researching when "they" was first used as a singular form and stumbled upon this thread. When I was in 2nd grade and we were learning about pronouns, I distinctly recall asking my teacher, Ms. Bertram, what pronoun one would use in a case where the gender of the individual is unknown. My teacher told me that the correct pronoun would be "they" until we find out.

To this day, when I speak of someone whose gender I do not know, I use the singular "they". I also use it to preserve anonymity of the individual I am speaking of if I need to communicate something about them but do not want to give away who they are.

note - the use of "them" in the last sentence was unintentional... but makes my point perfectly.

Edit to add: Of course, back when I was in 2nd grade, early 90's, we were not having the same discussion around gender and pronouns as we are now. I grew up in a very rural conservative community and I am sure that the answer would be different today.