To buy a gun in the USA you must be 18 or over. When you buy a gun from a registered dealer, you have a computerised background check. This only takes a few minutes.
If you buy a gun from a "private collector" (anyone who is not a registered dealer), you don't need to be checked. This effectively means anyone over 18 can get a gun.
The American Constitution, which sets out the country's rights and freedoms, says people are allowed to "keep and bear arms."