The abnormal physiology that manifests itself in healthy humans during their adaptation to the microgravity of space has all the features of accelerated ageing. The mechano-skel-etal and vestibulo-neuromuscular stimuli which are below threshold in space, result in an overall greater than 10-fold more rapid onset and time course of muscle and bone atrophy in space and the development of balance and coordination problems on return to Earth than occur with ageing. Similarly, the loss of functional capacity of the cardiovascular system that results in space and continuous bed rest is over 10 times faster than in the course of ageing. De-conditioning in space from gravity deprivation
 has brought attention to the medical hazards of deconditioning on Earth from gravity withdrawal as in sedentary ageing. Though seemingly reversible after periods of 6 months in space or its ground analog of bed rest.

It remains to be seen whether that will be so after longer exposures. Both adaptation to space and ageing do not merely parallel but converge as disorders of mechano-transduction. Like spaceflight, its analog bed rest telescopes
the changes observed with ageing and serves as a useful clinical model for the study of age-related deconditioning.  http://www.karger.com/Article/PDF/252852