The world record duration still sits at 438-days after 35 years. We limit people below that due to medical issues, suggesting no we don’t know how to safely do multi year space flight.
It’s possible people spending significant time on the surface of mars would recover, but that’s more speculation than proven.
The person who went 438 days was Valeri Polyakov, and he experienced exactly 0 ill effects from such, going so far as to intentionally briefly walk immediately after landing precisely in order to demonstrate that working on Mars after any transit would be possible.
Going beyond that is not really meaningful since that's far longer than any normal transit to or from Mars, which is the immediate target.
A young formally fit person being capable of making a few steps doesn’t demonstrate “0 ill effects.” He was vastly weaker upon his return as shown by making a few steps being considered a significant achievement rather than an actual sign of fitness like lifting a heavy weight and moving it around.
Living on mars at 38% earths gravity is believed to make things worse over time, so no you can’t just consider transit times independently. On arrival they would likely be fine inside a habitat. But trying to walk around in an Apollo 11 era 180 lb pressure suit in 38% gravity would be nearly as strenuous as walking around on earth and we’d like them to be able to work not just take a few steps and sit down. It’s possible to reduce that weight, but needing to carry oxygen tanks means there’s quickly a tradeoff between lighter weight and less time outside.
Now, for an extremely brief touch Mars and come back while burning a huge amount of fuel to make a shorter trip sure they’ll survive. But start talking a 3 year mission and things don’t look good.
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.
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.
It’s possible people spending significant time on the surface of mars would recover, but that’s more speculation than proven.