Yes, they do decondition faster. One interesting article asked what astronauts, seniors, and people with AIDS have in common. The answer is that all three groups tend to spend time immobilized, and it's that immobilization that.... If you have them in space, where they're not able to exercise and to use their muscles in the normal way, even the young, healthy astronauts start to decondition very fast. There have been several studies in which volunteers spent several days in bed; the deconditioning does happen, but it is faster for the older population.
On March 4th, 2008. See this statement in context.