It all depends on the law where you live.
Your parents will decide.
21
18 to purchase
If its a BB Gun, Then Yes.
You need to take this question up with the child's parents.
You must be 18 Years old to purchase one
18 years old
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
I would suggest an AirSoft BB gun. Not a Paintball or steel BB gun. Unless you have someone who is familiar with firearms then I would get him a Daisy Buck BB rifle and an adult to teach him the respect and rules of handling the rifle. The "Daisy Buck" is designed as a starter BB rifle for youngsters.