Humans age almost immeasurably more slowly in space due to time dilation. This is attributed to the high speeds at which they move as inhabitants of fast-moving vehicles. It is estimated that after a 6-month stay in space, a person will be 0.007 of a second younger upon return.
Time dilation is a component of the theory of relativity and says, in simplified terms, that a moving clock ticks more slowly the closer it approaches the speed of light than a stationary clock under observation. Common effects of aging, such as muscle atrophy and bone loss, are unrelated to time and affect people more rapidly in space than on Earth due to the lack of gravity.