I seriously cannot remember how young I was...but it was pretty young. I didn't mind one bit, cause I'd never fully believed in it all anyway. I even helped wrap presents as a kid (not my own of course).
But, the fun and spirit lasted years upon years because my younger brother (moderately mentally retarded) NEVER stopped believing...even if you tried to tell him! I think he may have finally stopped when he was about 20. But it was really cute, seeing him get so excited over Easter and Christmas. The only thing I didn't like was that he would wake me up around 4am to go lay out on the couch waiting for our parents.