It was beaten into me in school that this is incorrect. “They” is to be used as a plural pronoun only. It’s commonly used in the singular, but it’s wrong according to the English teachers I had. In referring to a person, you must choose either he or she under those grammar rules.
With that said, maybe it’s time for me to move into the future and accept that the meaning of the word has changed. I am confident those English teachers weren’t concerned about actual gender issues. Now, I think those issues are more important than the technical grammatical issues of English.
I’ve offended people in a social setting by insisting that this is the correct usage, when truly it was just me being autistic and informal rather than political.
Perhaps it was the English teachers who were wrong.
Correct or not, people have been using it like that for a while.