For most sports you have to be 16 to compete in the adult
Olympics. There are a few sports such as the marathon where you have to be
older (20 in this case) but you are never allowed to compete at a younger age.

This law was introduced to stop the exploitation of young athletes who in some
countries began training at a very early age and had to live under a very
strict regime. It hasn't necessarily worked - there are still plenty of young
gymnasts, for example, who train night and day, and of course there are plenty
of junior events for them to compete in, such as the Junior Olympics.

Some people think this law is unfair on young gymnasts especially, since these are often at their physical peak at the age of 14. But others say that it is quite wrong for children to compete in adult events and they should be free from the pressure to give a world class performance at such an early age.