One of the problems we run into today when looking at the Sphinx is that it has been altered considerably over the last couple hundred years by people attempting to "restore" it. Whole sections have been covered in concrete, stone has been added, portions of the body have been completely rebuilt, etc. The effect of these changes have been to give the statue a more uniform look, which undermines the theory that the body is older than the head. But look at this photograph:
This 1858 image of the Sphinx, taken before modern "restoration" attempts began, clearly shows the massive difference in the weathering between the head and the rest of the body from the neck down. The stone supports under the head and the stone cladding today blends the two, but this unmodified view really shows clearly that they are from different eras.
Besides, I still haven't seen a convinving explanation as to why the "buried" portion of the Sphinx was supposed to have weathered
faster than the portion that was exposed to the elements. I know the theories about groundwater saturation and such, but if there were any truth to them the foundations of the pyramids themselves should be crumbling by now (same stone, same age, same elevation). Because there is no evidence that this type of limestone degradation exists below ground anywhere on the Giza plateau, I'm inclined to believe that the weathering comes from a time when the deserts weren't so dry, and that it was rainfall that did it in. I don't know that I believe the theories that it's 12,000+ years old, but it's quite possibly the oldest existing human monument on the planet.