It is very easy to buy a gun in the USA. You must be 18 or over. When you buy a gun from a registered dealer, you have 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."
This makes it very hard to make laws limiting the way guns are bought and sold.