Yep 52, as I said still young. John Glenn flew a useful mission at 77 that’s old. Strength will often peak around 35 barring significant injury. Show me a 52 year old who’s happy to make a few steps and I’ll show you someone with significant impairment.

> stuck on the ISS for going on 9 months now

Stick right next to earth and all the medical care they would want on landing. That’s the difference they don’t need to be functional on earth.

> regain motor skills and coordinate

Meaning they’re nearly helpless for a significant period on landing. That’s a massive safety concern and limitation on mission profile.

> as well as strength/bone density

There’s serious concern around retaining let along regaining bone density on Mars. 38% g isn’t a well studied environment here but it’s a long way from earth and likely to result in significant bone loss up to a point. Meaning their trip back is now significantly more risky.