Well you know what they say.

As a friend of mine put it, "Who are 'they' and how come they know so much?"

I have come in for the usual obloquy from English teachers over my use of "they" in place of the more correct "he or she". Comments have been roughly evenly divided for and against. I can honestly say that I don't use it to excess but here is the OED website on the topic:

"The English language unfortunately lacks a simple singular pronoun which does not specify gender. Various people have suggested new words to fill this gap, but none of them have caught on, or (frankly) are ever likely to: it is not practical to try to change such a basic element of the language by sheer will.

"However, children and adults alike naturally find the obvious solution to this conundrum: rather than using the formal and awkward formula 'he or she', they simply use they, especially after words such as anyone and no one which are strictly singular but often imply a reference to more than one person.

"This is not a new problem, or a new solution. 'A person can't help their birth', wrote Thackeray in Vanity Fair (1848), and even Shakespeare produced the line 'Every one to rest themselves betake' (in Lucrece), which pedants would reject as logically ungrammatical.

"If you do not find this usage acceptable, there are alternatives. You could resort to the awkward 'he or she' formula, or to the practice of writing 'he' when you mean 'he or she' (which many people find objectionable), or to recasting all your sentences to avoid the problem!"

The OED website is a mine of information on this kind of issue.