In the twin paradox, why the slowing down of clocks can affect a person's growth?
Behind this problem is the (mis)understanding to the principle of relativity.
In section "The Postulates of Special Relativity (2)", we proved the principle of relativity mathematically. This kind of mathematical equality is absolute, its accuracy far exceeds one thousand digits after the decimal place.
As a side effect of the principle of relativity, we have no way to decide whether our reference frame is “stationary” or moving, without the use of an outside reference frame. Whatever the speed of our reference frame, all instruments' readings will be the same, and we human beings will not feel a tiny bit of difference.
By the theory of special relativity, a clock in a high speed spaceship runs slower. A man on the spaceship has to grow older more slowly, else he will detect the clock’s change in tempo, and the principle of relativity will be violated. This is the reasoning why a person on a spaceship can be much younger.
Actually the slowing down of a clock and the slow growth of a person in a spaceship are all against the principle of relativity.
According to the principle of relativity, a physical law is not affected by the selection of reference frames. No matter an apple falls from a tree to the ground, or falls to the floor in a train, the falling is always caused by the same gravity, and has nothing to do with the speed of the train.
If a clock really runs slower, and a person really grows slower in a spaceship, that is because the physical environment there is different. If you really want to link them to the speed of the spaceship, we can say that the high speed motion of the spaceship might affect the physical environment. But these effects are unknown, and have nothing to do with the equations of special relativity.