The faster someone travels, the slower their body ages. This means that a guy orbiting the earth (in the same direction earth is rotating, preferably) will age more slowly than the one standing still...and will be younger, even if the dudes happened to be born at the same time.
This has to do with the speed of light being relatively equal to everyone...no matter their speed. In other words, if there's a guy sitting still...and a guy traveling at 1/2 the speed of light away from the guy sitting still...and a guy behind the guy sitting still fires up a super powerful flashlight, the speed of that light will seem to be exactly the same to both the guy sitting still & the guy traveling 1/2 the speed of light away from the source. By the same token, if there were a guy traveling 1/2 the speed of light toward the super powerful flashlight, the light's speed would appear to be the exact same to him.
Since each person observes this speed to be the same irrelevent to their personal speed, each person has a "personal time." Since it all is observed the same to each person, it takes the same amount of time for light to be 1 "light second" ahead of the person. This means that the speed of light is the base of each person's relative time and that a universal time cannot exist.
I saw a youtube video on this awhile ago that made this a lot easier to understand with visuals and whatnot. I'll try to find it later if no one else posts something of that sort.
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