"The alternative to the masculine generic with the longest and most distinguished history in English is the third-person plural pronoun. Recognized writers have used they, them, themselves, and their to refer to singular nouns such as one, a person, an individual, and each since the 1300s. For example, in 1759 the Earl of Chesterfield wrote, “If a person is born of a…gloomy temper…they cannot help it,” and, echoing this sentiment, W. M. Thackeray wrote in Vanity Fair in 1848, “A person can’t help their birth.”

"Modern writers of note, from George Bernard Shaw to Anne Morrow Lindbergh, have also used this construction, in sentences such as To do a person in means to kill them and When you love someone you do not love them all the time. The practice is widespread and can be found in such mainstream publications as Christian Science Monitor, The Washington Post, Discover, and Wall Street Journal.

"The use of the plural pronoun to refer to a singular noun or pronoun is hardly restricted to writing, however. Its use is so common in speech as to go without being noticed."

~http://www.bartleby.com/64/C005/018.html