One way to think of prednisone that a collaborator brought to my attention is that it causes clinical signs of aging at both an organismal and cellular level. Why this happens is fairly uncertain, but things like osteoporosis, muscular wasting, shifting of the fat pads, are all things that happen as we get older. It is an active area of research, but the system is so complex it is hard to tease answers out of.
As to how long someone can be on it...I'm not that certain. I would imagine for their entire life, but it leads to weak bones and an inability for the skin to heal from wounds due to decreased collagen production. These people probably just appear to be older than they really are and suffer from maladies related to age moreso than people of their age group.