They has a long and noble history in English, going back to at least the 1200s in writing and certainly much further back in the spoken tongue. And as early as the time of Chaucer, they was used to denote a singular person. Shakespeare used it that way too. More recently, the American Dialect Society has given their imprimatur to singular they.
Just as singular they is finally starting to get its fair shakes, a new use is arising: the genderless pronoun. Read more about that here: ‘They’: the singular pronoun that could solve sexism in English | Books | The Guardian