According to the theory of special relativity, twin B would be younger than twin A upon their reunion. This effect is known as the "twin paradox."
In the scenario you described, twin B travels to Alpha Centauri (AC) at a relativistic speed, while twin A remains on Earth. Due to the high speed of twin B, they experience time dilation, which means that their clock runs slower relative to the clock of twin A on Earth.
When twin B returns to Earth after their journey to AC, they will find that less time has passed for them compared to twin A. This is because time dilation causes the moving clock (in this case, twin B's clock) to "tick" more slowly. Twin B's slower experience of time results in their younger age compared to twin A.
It's important to note that the twin paradox is not a true paradox in the sense of a logical contradiction. It arises because the situation involves a comparison between two different frames of reference: the inertial frame of twin A on Earth and the non-inertial frame of twin B's accelerated motion. Special relativity provides a consistent explanation for the observed time dilation effects in these scenarios.