Singular "they" dates back to the 14th century. "Thou" fell out of use in the 17th century. https://www.oed.com/view/Entry/200700 https://www.oed.com/view/Entr

Singular "they" dates back to the 14th century. "Thou" fell out of use in the 17th century. https://www.oed.com/view/Entry/200700 https://www.oed.com/view/Entry/200700 https://www.etymonline.com/word/thee#etymonline_v_10720 https://www.etymonline.com/word/thee#etymonline_v_10720