Actually it’s older than people think. Shakespeare used it for stuff like “Every knight grabbed their sword”, and even for talking about a specific person it’s not a new phenomenon to use singular they if the gender doesn’t matter (so I was told in a linguistics sub over on r*ddit when I insisted it was new)
The only new thing is that people say, it’s their prefered pronoun.
When I was young, I was poor. But after decades of hard work, I’m not young anymore.