Once Upon a Time... Life

What's the name of the creature that stands on four legs in the morning, two at noon and three in the evening"? This is what the Sphinx once asked. It's the human being. In the morning of his life he crawls on all of his four legs. During the main part of his life he walks on two and when he grew old, he needs a walking stick and thus walks on three legs. Why are we getting old?