Treating us like Juveniles

At the ripe age of 18 a person can legally get married in the United States of America. If that person's guardian signs a slip of paper the ceremony can take place at an earlier age. That tells me that according to our government we are ripe and mature enough to make a decision about whom we want to spend the rest of our lives with. At the age of 16 we can operate a vehicle which can be potentially dangerous not just for us, but most certainly will place us into traffic situations in which a wrong move can cause harm to others. At the age of 17 we are able to sign up for military service, and be provided with or put in charge of deadly weapons.
If our government considers that we are ripe and mature enough by the time we live to be 18 years of age to provide us with all of the above mentioned freedoms of choices, why are we not considered responsible enough to make our own decisions on issues such as alcohol consumption and marijuana use? If we want to get scientific about the age at which an individual can make important decisions for him/herself then I seriously doubt that we would come to the conclusion that we're responsible enough for the above mentioned decisions at different times of our lives.
Stop treating us as if we were a nation of juveniles! But then again...!