You age less because less time has elapsed in your reference frame compared to the time that had elapsed on Earth. The rate at which physical/mechanical processes take place is slower as you approach the velocity of light when compared to the rate at which these same processes take place on the relatively slowly moving Earth. The slowing of time as you approach the velocity of light is only apparent from an external reference frame that does not share the same accelerated motion.
A large gravity field has the same affect of slowing time as witnessed from an external observer not subject to the same gravity. To an external observer watching an object approaching the event horizon of a black hole, the objects progress toward the black hole would appear to slow down, the wavelengths of light emitted or reflected by the doomed object would stretch toward the infrared while the lights intensity would fade to black at the instant its apparent motion ceased. For an observer experiencing this journey into the event horizon, looking back from whence he came he would observe an acceleration of events in the universe he was leaving concluding with the end of time for both it and himself.