If u use a lot of sugar in ur coffee it can cause u to become a diabetic early in life! The reason being is that in the morning there is no food in ur belly to absorb all that sugar and it stays in ur belly!
Coffee will keep you awake for longer than you should which causes you sleep less when you should have enough sleep. I would recommend to drink coffee only when you have to stay up late to do your homework or something important or drink coffee before you go to work if you feel sleepy. But i dont recommend to drink coffee at early age because its not good for your health, and when you drink too much coffee you will become a coffee addict when you grow up, which is not good.