It's up to your parents and the laws of the country where you live.
Your parents will decide.
21
It all depends on the law where you live.
18 to purchase
If its a BB Gun, Then Yes.
You need to take this question up with the child's parents.
18 years old
You must be 18 Years old to purchase one
Depends on your skills, size and parents.
No
It's up to your parents
You have to be 18 to purchase a BB gun but you can use one under adult supervision or guidance at any age as long as you act mature and understand the basic safety rules of handling a BB gun or Rifle