If a human's life is desrcibed in terms of a day, then the morning is life as a baby ( ie, crawling - 4 legs ), two legs at midday - walking upright, three legs - walking with the aid of a stick, and unable to walk is the evening of your life ie, elderly/infirm.
cause in the morning of his life (as a child) man crawls, at midday (in the prime of life) on two legs, in the afternoon (old age) with a cane, and in the evening (either dead or infirm) in a wheelchair.