From infancy youth is reached, and from youth, old age. As the creature advances from one stage into another, the from shown in the previous stage becomes changed.
कौमाराद् यौवनं चापि स्थावीर्यं चापि यौवनात्। अनेन क्रमयोगेन पूर्वं पूर्वं न लभ्यते॥