The person in the space ship will experience time twice as slow as people on earth. So the person in the spaceship would expect people on earth to age twice as quickly.
I targeted this part of your reasoning. Time on spaceship is moving slower (in a sense) than time on earth in reference frame where earth is stationary, yes, but it doesn't follow that time on earth therefore moves faster than time on spaceship in reference frame of spaceship, quite opposite.
t'=\gamma(t-vx/c^2)
It is both valid when t is measured in reference frame of spaceship and in reference frame of earth.
Thus time in reference frame of muon is moving slower relative to our reference frame and time in our reference frame is moving slower relative to muon's reference frame.
It has been suggested that animals have less subjective experience than people. For example, it would be possible to have an animal that counts as half a human for the purposes of morality. This is an argument as to why that may be the case.
If you're moving away from Earth at 87% of the speed of light, time dilation would make it look like time on Earth is passing half as fast. From your point of reference, everyone will live twice as long. This obviously won't change the number of life years they live. You can't double the amount of good in the world just by moving at 87% the speed of light. It's possible that there's just a preferred point of reference, and everything is based on people's speed relative to that, but I doubt it.
No consider if their brains were slowed down a different way. Suppose you uploaded someone, and made the simulation run at half speed. Would they experience a life twice as long? This seems to be just slowing it down a different way. I doubt it would change the total amount experienced.
If that's true, it means that sentience isn't something you either have or don't have. There can be varying amounts of it. Also, someone whose brain has been slowed down would be less intelligent by most measures, so this is some evidence that subjective experience correlates with intelligence.
Edit: replaced "sentience" with the more accurate "subjective experience".