Time travel, either back in time from the journey location to the return arrival time or along their personal timeline. If the "traveler" twin is displaced in time as well as space, or goes somewhere where the passage of time differs from that on Earth (in this case it would have to go faster), then they may actually experience more time than the "stay home" twin.
In many ways time is just another dimension, we humans can't traverse it with modern technology, in fact if our current understanding of physics is correct the universe can only traverse it in one direction but if the traveler is being assisted by ASBs it may be a different story. The mode of transportation could also be a source of temporal displacement in the case of a Traversable wormhole since the time-like dimension of the wormhole exit is not necessarily fixed. Either of these scenarios will result in the traveler being older than the stayer if they spend more time elsewhere than the twin spends on Earth during their trip, even if the two are experiencing time at the same rate.
We also know that small changes to our experience of time occur when we leave Earth's gravity well, but they're microscopic, we don't know for certain that there are not places where that difference is far more pronounced, although current evidence suggests not, a trip to such a location would also have the desired effect.