unfair

Does anyone besides me ever stop to think that its unfair for people under the age of 18 to be tried as adults. Obviously I'm not, but you'd think it'd be a bigger issue.
So you take someone thats lets say 14 years old. Lets say they kill someone. Lets say they are fully knowledgeable of their actions, even though there is no real scientific way to prove that, but lets say for the sake of the example that they are. So the court decides they should be considered an adult.
But wait a second. If they're an adult, why can't they drive? Why can't they vote? Why must they be in the charge of someone over the age of 18? I think the cruel and unusual punishment thing applies to this. Wouldn't you say its cruel for someone to have to face the consequences of being something, while getting none of the benefits of said something. It's like soldiers not being able to drink. If they are able to serve in the Army, I say they should be allowed to drink legally. Thats just my opinion. But its the same principle. You can't have some get punished according to standards of another group, when in all other areas, they are not a part of that group