They did at one time only refer to more than one person, which Silvanus said wasn't the case.Singular they - Wikipedia
en.wikipedia.org
"This use of singular they had emerged by the 14th century, about a century after the plural they. It has been commonly employed in everyday English ever since and has gained currency in official contexts. Singular they has been criticised since the mid-18th century by prescriptive commentators who consider it an error."
So the last 700 years, then.