Good question. It's difficult to imagine someone being completely apathetic, but if one were, I don't think it could last a lifetime. Something will pop up to change that, even for a short time: a birth, a death, a new job, a new love, a child, a pet. We may experience a certain level of numbness to many things happening around us, but I don't think anyone is so numb as to not be affected by anything at all, forever--unless you're brain dead, literally. Maybe if we truly cared about nothing, our brains and body would eventually shut down for lack of stimulation.