Growing numbers of people, including some transgender, nonbinary, agender or gender-fluid people, use they/them/their as a gender-neutral singular personal pronoun. 

This has increased the usage of “they” in writing as a gender-neutral singular pronoun when gender is unknown to avoid the awkward “he/she” or even worse, assumed male pronouns. 

The AP Style Guide has enshrined this guidance: 

• When necessary, use they rather than he/she or he or she for an unspecified or unknown gender (a person, the victim, the winner) or indefinite pronoun (anyone, everyone, someone). But rewording to avoid a pronoun is preferable. 

But there’s another school of thought, put forth by Charles Babington, my Associated Press colleague, in his fantastic writing tips newsletter (subscribe by emailing clbabington@gmail.com):

• “Grammar rules evolve, but when a trend goes too far, it’s time to resist. Such is the case for the widening use of “they” and “their” with singular subjects, as in, “When someone sneezes, they spread germs.” I generally agree we should avoid the ungainly “he or she.” But there are better ways to avoid “someone… they”…(Note I’m not talking about someone who uses the pronouns ‘they’ and ‘them.’ Writers should honor their choices).
• Such as: “When people sneeze, they spread germs.” “Someone sneezing spreads germs.” (Better still: “Sneezing spreads germs.”) These are easy lifts.
• “Seems to me the rule should be: Try to find feasible alternatives to using “they,” them” and “their” with singular nouns. Surrender only if you must.” 

So what’s a writer to do? Whichever school you ascribe to, it seems to us you can’t go wrong as long as you abide by the following:

• However you use “they,” make sure your meaning is clear.
• Respect a person’s pronouns. 
• Don’t make assumptions about a person’s gender identity based on their pronouns— or vice versa.