The answer to the riddle "What goes up but never goes down?" is "age." Age is a measure of the elapsed time since a person's birth. As time can only progress and cannot be reversed, age must only increase.
Even in extreme cases, such as a person moving at speeds near the speed of light, age does not decrease. A person moving at relativistic speeds ages more slowly than a person moving at a slower rate, but she does not become younger.
In Einstein's Twin Paradox, the parents of twin 10-year-old boys keep one son on Earth and send the other on a round trip to a star 25 light years away. When the traveling twin returns 50 years later, he has only aged six months, making him 49.5 years younger than his twin brother.