[A.J.Armitage]

And he has the right to purchase beer, if the state he chooses to reside in views 18 year olds as mature enough to handle it.

No, he has the right to purchase beer, if he can find a willing seller. No one's rights depend on whether the government thinks he can "handle" it.

[Texaggie79]

No one's rights depend on whether the government thinks he can "handle" it.

No, it depends on whether the people of the community/city/state he chooses to live in feel safe with allowing such young people to have access to such dangerous things.