So if you're traveling near the speed of light, time moves slower. If you go on a long trip at near-light speed and come back, you could've been gone for what appeared to be a week to you. But to everyone else on earth (moving at normal earth speed), you were gone for years. So you've aged one week over the course of a few years. Keep traveling all over the place at relativistic speeds, and you can prolong your life indefinitely. You'll still age at what seems like a normal pace to you, but to people sitting around on planets it will seem as though you hardly age at all.