General Relativity Question

I've recently learned about general relativity from my Physics professor, where English is his second language. Needless to say, there are parts of it that don't make sense, so I'm hoping someone can explain this next question.I was given the following scenario (which I modified slightly): Consider an astronaut who has a twin on a planet, and relative to each other, they are stationary. Their speed relative to the inertial reference frame of the Universe, however, is unknown. The astronaut travels to some arbitrary point in space and back for what he thinks is a year, but upon arrival he notices that his twin has aged 10 years. This happens because he approached the speed of light on his trip which warped the effect of time on him, but obviously not the planet.Question: Why would this happen? If the astronaut approaches the speed of light, he is doing so relative to the speed of the planet. However, from the perspective of the astronaut, the planet would be moving nearly the speed of light. Given this logic, his twin should've only aged a month.Possible answer: I've considered that this is because of the speed of the astronaut based on the inertial reference frame of the two bodies. If this is right, then I have another, slightly more complex problem.Scenario 2: Consider 3 bodies that are stationary relative to each other. Body 1 remains stationary, but bodies 2 and 3 group together and take off with the same relative speed and orientation. At an arbitrary distance, body 2 heads back to body 1 at a speed half of what it traveled previously.Problem: According to the inertial reference frame of the 3 bodies, body 1 should be aging faster than body 2, which should be aging faster than body 3. However, taking the inertial reference frame set by bodies 2 and 3 after they finished accelerating, body 3 should be aging faster than body 2.

I usually get this type of answer, but the detail that you went into was especially helpful in understanding the first question. The only part that remains vague is what it means for acceleration to ruin the inertial reference frame.The bigger question comes from the second part: Speed is relative to what? I'll explain precisely what I'm asking through this next scenario:
Goku is launched in a rocket away from Earth, which after 1 hour travels 80% the speed of light relative to the planet. Out of boredom, he decides to play catch with Gohan, and reaches into his pocket to grab his indestructible baseball. After he powers up a couple levels, he chucks the ball at 50% the speed of light back at the planet.First glance: The Earth might’ve aged 25 hours, but it appears as though the space ship has only aged only 15. When the ball is thrown back, it never reaches the Earth because it is still traveling away from it at 30% the speed of light. After the ball is thrown, we have that the Earth (stationary) is aging faster than the ball (30% speed of light), which is aging faster than Goku (80% speed of light). What are these speeds relative to? The initial inertial reference frame.Second glance: After the rocket hits full speed, Goku and the baseball set a new inertial reference frame. Throwing the baseball will make it travel 50% the speed of light relative to Goku, so the ball seems to be aging slower than Goku. Taking the planet out of the picture, we have that Goku (stationary) is aging faster than the ball (50% the speed of light). These speeds are relative to the new inertial reference frame.I hope this better explained why I’m confused. It doesn’t seem that the presence of the planet should determine whether or not the ball ages faster or slower than Goku, but it doesn’t seem that you should only be able to take into account the initial inertial reference frame. I’m thinking either that the answer lies in the acceleration ruining the inertial reference frame, or my misconception of apparent aging.

How interesting. I'm gathering that aging itself is relative, which would definitely solve my paradox. Thanks for your help!