Many Egyptologist (including Dr. Zahi Hawas, former Minister of State for Antiquities Affairs) insist that Sphinx was built nearly 4500BC, by the builder of 2nd pyramid of the .i.e. the pyramid of Chephren (also read as Khafre, Khefren and Chephren).
But other researchers like John A. West and Robert Bauval (who first put forwarded the “Orion correlation theory”) believe that the both pyramids & Sphinx are much much older then Egypt civilization. Here I’m summarizing some extracts from their research paper where they are trying to prove that Sphinx is atleast 10 millennium BC old.