18 is generally considered the age by which you are old enough and mature enough to be considered an adult and to make serious decisions regarding your own health and future.
... if the government and the country viewed adults and minors as having the same levels of cognitive reasoning to determine what is and is not the correct course of action, then there wouldn't be age restrictions on many things.
The age of majority is essentially an arbitrary number (as a thought experiment I'm sure we all know 17 year olds who are very smart and capable, and we all know 30 year olds who are fucking morons). It is this way because it is simply easier
than if the courts & laws had to be judged on their own merits for each individual case.