You (or ye) as a singular pronoun started in the late fourteenth century, originally as a term of respect. Check your Shakespeare; you'll note that when characters are speaking up, they use you or ye, when they are speaking down they use thee of thou. Use of they in the singular dates from the early sixteenth century. Incidentally, there was a time when it was considered proper to say, e.g., you were when the you was plural and you was when it was singular.