Not only do astronauts come back with weak muscles and frail bones… But they also suffer from skin thinning, atherosclerosis (stiffer arteries), resistance to insulin and they suffer from loss of vision due to cataracts many years earlier than expected given their chronological age. These symptoms look a lot like skin aging, cardiovascular aging, age-related diabetes and so forth. In fact, it is pretty accurate to say that astronauts age at an accelerated rate. This is despite the fact that the current generation of astronauts follows a rigorous exercise program.
Interesting stuff. To keep in top form, the human body needs resistance. I've sibscribe to the notion that a decent part of aging is actually "exercise deficiency". Check out this 90 year old gymnast.