Special Beer Service

Why I have to register for special services, but I can not drink?
I mean, if Canada invades Im going to have to die for government so how can the government did not give me my God (and constitutional) law of reason when it comes to war to die I'm a man and when it means have a beer I'm a child, can someone explain this to me?
I think the idea is to have finished growing physically at 18, but the brain remains developed until 21. I also believe that the drinking age should be 18, but apparently many people do not agree because that is the law. The good news is that if you are in the military, usually does not get carded, and the police can not arrest you for underage drinking.
A Public Service Announcement From The Motor City Machine Guns