This is a job for robots and especially with new AI advances. Trying to fit the biology evolution created for Earth for non-Earth places is a classic square peg problem. I've always sort of mused that the 'gray alien' body is some sort of collective mythical idea of a space-engineered body. That is to say, if you want biological beings to casually work on other planets and moons, our Mark I primate bodies aren't good enough. Ethically, breeding 'space body children' should be a no-go for any civilization that values ethics and consent.
So that just leaves us robots, which not only makes scientific sense but ethical sense. Even the "old man" solution is unethical. We cannot know which one of these men might live to 100 if not bathed in radiation. Estimates on average age don't apply to the individual, so sending a 70 year old who will "die anyway in his 80s" is still very problematic. Fun fact, William Shatner is 92, so the famous Captain Kirk actor is one of those long lived individuals. Imagine sending him to Mars in 2004 at 72 because "he'll die soon anyway," but instead he's starting to look like someone who is going to live well into his mid or late 90s.
Its probably worth mentioning that when humanity does figure out a cheap and safe flight to places like Mars, Venus, moons of Jupiter, etc and figures out the engineering and bioengineering, we'll probably either be at our extinction event before then or AI will be at the level where we'd be just as much machine as human, or entirely replace by self-aware robots. So its a bit like those old film strips where they launch a manned cannon ball to the moon or an airship to go to Venus. The society, tech, and culture, etc of the time just wasn't far-seeing enough to understand the complexity of these issues and the changes needed to address them. We never danced around the moon's surface in Victorian gowns, used a sextant to navigate our spaceship, or brought a contingent of slaves to Mars. Modern thinkers are making similar mistakes as these naive old stories. Migration to the stars is just not going to be done in these bodies nor with this technology.