There's a base right, but it's qualified by state. Something I don't think a lot of people outside the US understand is, pretty much everything varies by state. There is a federal minimum for most things, then each state decides from there if they will increase that or not (minimum wage/employee rights are the most obvious ones). Then sometimes cities will have even stricter laws. But city laws can't negate state laws, which can't negate federal laws (generally).
For guns, in my current state of North Carolina, I can pretty much go into a shop and walk out with a gun, no different than if I wanted to buy a skateboard.
In my previous state of New York, I could buy a rifle or shotgun without a license, but for a handgun, I'd need a license. If I lived in NYC, I need a license for all of those. And the process to get that license includes a background check, multiple proofs of citizenship, my legal history, any military history and reason for leaving if I left, information of any business I may run, and two character reference letters that have been notarized.
The minimum age also varies by state and type of gun. The youngest is 14, while the oldest is 21. Most states it's 18 or 21. In North Dakota, there is no minimum, but minors have to be supervised while using a gun.