Acording to law of physics, the shorter an object is, the more is stability because the centre of mass can be easily balanced. Therefore, a shorter  person will be more stable compared to the taller during the performance in gymnastics .

Usually, a younger person is shorter than the older. Therefore it is advantageous in gymnastics if a team can use younger kids unlike other sports where older will perform better. The issue of underage girls arises due to this reason not only because of  age limt.