And what makes the transition from teenager to adult the age of 21...there is not set AGE when your body is finished maturing. Some may be finished maturing at the age of 18, while others may take as long as till the age of 24.
If I am capable of voting for elections at the age of 18, driving a car alone at the age of 17, and buying cigattes at the age of 18....what's the difference? If you can't trust a person with alcohol, you can't trust them with a car. Or with the future of the country in their hands.