What happens to really old people in jail?

People get sentenced to life in prison all the time or when they are 30 they get 65 years. But when I watch those prison shows like Lock Up on MSNBC, I never see any old people. Sure you see people in their 50s or 60s, but what about people in their 80s or 90s? I know they get medical care in prison, and I know they are fed, so I doubt they all die by the time they are 60-something. Where do all the old prisoners go or do they stay in prison?