“In physics, the twin paradox is a thought experiment in special relativity involving twins, one of whom takes a space voyage at relativistic speeds and returns home to find that the twin who remained on Earth has aged more. This result appears puzzling because each twin sees the other twin as moving, and so, as a consequence of an incorrect and naive application of time dilation and the principle of relativity, each should paradoxically find the other to have aged less. ”
This problem could be worked out with General Relativity, as it includes acceleration as part of the space trip. However, Special Relativity suffices.
In fact, the travelling twin is undergoing acceleration, which means they are not in a inertial frame of reference. This invalidates the application of Special Relativity from the perspective of the travelling twin. Therefore, the perspective of the stationary twin is the correct one. Time is dilated for the traveller and they age less.