Sphinx is at least 20,000 years old. According to Hancock and others. :-)
John Anthony West recruited geologist/geophysist Professor Robert Schoch to date the Sphinx. On page 287 of Schoch's book, Voyages Of The Pyramid Builders he states:
"...for this reason I come back to my estimate of circa 5000 to 7000 BC for the oldest portion of the sphinx."
It is Schoch's studies than Hancock quotes too.
Click on the link and scroll down to the editoral review. Very unique and interesting book.