Suppose wrlwind gets in a spaceship and heads out on an extended voyage (and climbs the walls) while ducknut stays behind on earth and climbs on rock. Wrlwind accelerates at at 1 G, because he's used to earth's gravity.
As his ship approaches 0.98 times the speed of light, Ducknut notices that wrlwinds rate of acceleration seems to be slowing. Meanwhile wrlwind still feels that he is accelerating at 1 G. From Ducknut's frame of reference, the mass of wrlwind's ship in increasing and his clocks are speeding up, but as far as wirlwind is concerned his weight, or mass, is the same and his clocks, as well as his perception of the passage of time are the same.
So if wrlwind goes fast enough, say above 0.99x C and travels for long enough, he might eventually come home to find himself much older than ducknut, even though they may have been the same age when he left.