According to law of Biology (actually there is no such law but just common sense), the younger the person less stable they are.  If they are more stable why don't you put toddler in the gymnastic.

Secondly , i believe, the hand, leg everything should be strong enough to hold the bar, grip it. Age 14 is not the one where gymnast gets an undue advantage.

Whatever, may be the age that should never be the criteria for allowing or disallowing any players. Its like saying tall guys are not allowed in Basketball because it gives them the advantage. If IOC is concerned about the injury that may be different reason.