Joozey, I Don't get it. In my eyes, what you said doesn't answer my problem.
If I drive away in a car, you'll say I'm accelerating. But then I tell you that relative to me, you're accelerating in the opposite direction. And, apart from the direction, your acceleration is exactly the same as mine. So we should both age slower, and both age "the same amount" slower.

Joey, I might find some time to look at the link, right now I'm in a rush...