I see no reason to change the law. There are far too many irresponsible 18 year-olds out there.
Yes, it seems inconsistent that 18 year-olds can join the military and vote. They can shoot a fully automatic rifle in the Army at 18 years old, but they have to wait until they are 21 to buy a handgun, or handgun ammunition, as a civilian. Makes no sense to me.
Yes, they will have access to alcohol before they turn 21... and even before they turn 18 - no matter what the law says. They also have access to all kinds of drugs - from middle school on. It doesn't make it right.
Parenting is the key. Teaching right from wrong.