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Minimum age
Because you use weight during a training, the bodybuilding stress the articulations. So begin this sport to young can be bad for the growing of a kid.
So I think that at 16 years old, you can begin this sport but not to hard. Begin with basically movements and don't use too heavy weight.
It better for the future, at 18 years old, you can really begin, and so you'll all ready have the base.
Dangers
There are three kinds of danger.
Hurting the tendons:
With a bad warm up, you can hurt your tendons.
Hurting the muscles:
It's coming from a too heavy weight, a bad warm up.
Breaking the bones:
A weight fall on the food, and with 20kg on the food, you'll done laugh.
It's coming from a too heavy weight, from the fact that you don't listen your body.
Don't presume of your forces
Listen to your body. He knows better than you how he fells.
Don't train your back if you have back pain, don't think you'll have a good train if are tired.
Don't listen to your body, and you can hurt yourself.