In the State of Oklahoma, you have to be over the age of 18 to buy a handgun from a non-federal firearm licensed dealer. Under federal law, you have to be over the age of 21 to buy the gun from an FFL. To buy a long gun from an FFL, you have to be over the age of 18. To possess a firearm (either a handgun or a long gun), you have to be over the age of 18.
If you are under the age of 18, there are exemptions, such as if you are under adult supervision, using a firearm for recreational purposes at a shooting range, while hunting, or with educational type of activities, such as ROTC.
The State of Oklahoma has preempted all firearms and ammunition laws. That means that municipalities in the State of Oklahoma cannot create laws that are in contradiction to the Oklahoma Self Defense Act or Oklahoma Firearm Laws.