Where I live (Canada) you can get your unrestricted firearms license at the age of 18. Unrestricted classifies as most guns used to hunt (ex. shotguns, rifles). You can get a restricted license which allows you to purchase handguns. This is for Canada though so the laws where you live are probably different if you live in the US. Try researching your local laws on firearm possession and purchasing.
Your parents will decide.
21
If its a BB Gun, Then Yes.
It all depends on the law where you live.
18 to purchase
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