3. I have been training in karate for over a year and I would like to improve my skills by using a punching bag. I am 14 and I read that it is unhealthy for young adults to use heavy bags because it can effect bone structure. Is this true and if it is what can I do when we use the bag during class? -

... You are wise to exercise some caution and restraint when it come to the use of the heavy bag. We must remember that the bag is only one of many training tools and there are a variety of exercises that will assist in the development of the martial artist.
... Speed, accuracy and distance are ultimately more important in improving technique and power than just hitting hard. The risk to joints and the spine from repetitive stress injuries is real especially for young people and those in the early stages of physical conditioning.
... Moderate use of the heavy bag can still be productive if you concentrate on percussive strikes rather than blows that attempt to force through the bag at full power. Imagine striking a large drum with a clean precise stroke that resonates rather than smashing it with a blow that could break the drumskin.
... There are other ways to develop strength and exercises that do not involve impact. Your health and an injury free future should not be sacrificed. Developing control and direction of intention will be far more useful to you as a martial artist than the ability to pound a heavy bag.