Shakespeare used "they" with singular antecedents. So there! > "Because when a construction is clearly present several times in Shakespeare's rightly admired plays and poems, and occurs in the carefully prepared published work of just about all major writers down the centuries, and is systematically present in the unreflecting conversational usage of just about everyone including Sean Lennon, then the claim that it is ungrammatical begins to look utterly unsustainable to us here at Language Log Plaza. This use of they isn't ungrammatical, it isn't a mistake, it's a feature of ordinary English syntax that for some reason attracts the ire of particularly puristic pusillanimous pontificators, and we don't buy what they're selling."
(Via Language Log.)