Polyakov was 52 years old when he did his 438 day stay, and astronauts always take a couple of days to readjust to gravity. The overall effects of well over a year in space seem pretty comparable to a few months in space. It's just not a big deal. Those Boeing astronauts (both around their 60s) have been stuck on the ISS for going on 9 months now, and NASA's basically like ¯\(ツ)/¯.

Back on Mars they'll be able to quickly regain their motor skills coordination, as well as strength/bone density. So it will be effectively equivalent to Earth in this regard - actually it will be even better than Earth because the reduced gravity will probably send the overall adjustment time down from a couple of days to a couple of hours, especially with a normal duration transit.