Good example, especially since the states have no business telling anyone what guns he can buy.
Why do you have to be 25 to rent a car? I know it's not a law, but it is a standard.
It can be an industry standard all you want, but it's not comparable to a law. Why? Because the car companies own their cars. They can, or ought to be able to, refuse to rent to anyone for any reason or no reason. That's because they have property rights. An "underage" drinker also has property rights.
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.