That's a complicated one. As I recall, you don't get the adult rate on car insurance until 25 or something (not a law, of course), you can't run for US Congress until 25, can't run for U.S. Senate until 30, can't run for President until 35, etc. There are all sorts of seemingly arbitrary age limits. It'd be really hard to say a person should have all those rights at 18, or should have none of them until they're 35.
You get an “adult” rate on car insurance at any time you start driving. It’s just that your rates will drop when you turn 25. I see that as completely different. Kind of the same thing as when you reach a certain age, your health and life insurance rates start going back up
The ages to run for President or Congress are set in the Constitution.
I still say that there are certain rights and responsibilities that come with being considered a legal adult. I believe once you reach that age, you should have those rights and responsibilities. If you are still considered a “minor” at 18 because you can’t drink or purchase a firearm, why should you be considered an adult and be sued or sent to prison?
If people don’t think someone should be an adult until they are 21, then make that the rule. Below 21, you are a minor. You don’t go to prison, you can’t sign a contract, you can’t drink, you can’t buy a firearm. But if the age in which you become an adult is 18, then you get the benefits and downfalls of being an adult at that age.
I don’t see how someone can be an adult when they turn 18, but then be denied the RIGHT to keep and bear arms until they are 21. Regardless of whether one thinks an 18 year old is responsible enough to own a gun at 18, if they are legally an adult at that age, you shouldn’t be able to deny them that RIGHT. Remember, the RIGHT to keep and bear arms is a right, not a privilege.