From which age you are allowed buy a Weapon in the USA ?

What kind of a weapon (handgun, rifle, knife, grenade)? For what purpose (antique collector, hunter, protection)?

The minimum age to purchase a handgun or handgun ammo is 21.

The minimum age to purchase a rifle or shotgun (or ammo) is 18.

This is federal law... each state may take it a step further. Certain state may allow someone under 18 to own or use a rifle for hunting or at a shooting range if a parent is present. There are also different age minimums for purchasing knives that vary by state.

It usually varies by state law. 18 is the youngest (with parental or legal guardian consent). For handguns, 21is the minimum age. BB guns and similar items are not considered firearms. Knives, stun guns, and similar items vary by state law and it is best to consult with a licensed firearms dealer. The topic can be a bit complex depending on the individual situation.

While SarahT is correct as far as purchasing a firearm from a FFL dealer the age that you can purchase a firearm from an individual is set by state and can be lower than the federal law.