There are competing hypotheses on how old it is/how it was made. I favor Peter Lacovara’s hypothesis that the body of the sphinx is nothing more than an outcropping of stone from a quarry that was used to build the Great Pyramid. Not wanting the outcropping to go to waste, Khufu commissioned the project. The weathering marks are thus not weathering marks but tool/quarry marks that have eroded over time. This would make the Sphinx roughly the same age as the Great Pyramid.
I do not think there is any hollow space below the Sphinx since it is on/made out of bedrock.