Picture an inflating balloon. The outward expansion of the balloon represents a 2-dimensional universe undergoing the passage of time. As the 2-dimensional universe expands outward, it carries all of these bodies along with it. The movement of these bodies outward at a slower rate represents the decrease in the rate of aging that the bodies undergo. The more massive the body, the more slowly it ages.
All objects possess mass.
As a result, the object in motion is carried outward at a slower rate, and in effect ages more slowly. We can conclude that the faster the rate at which an object travels, the more slowly it ages.