you have to be 21 or older to even own a handgun.
You are 100% wrong. You have to be 21 for a dealer to sell you a handgun, but not to possess one.
18 to possess a handgun
13 (if I remember correctly) to possess a long gun
OCGA § 16-11-132 (b) Notwithstanding any other provisions of this part and except as otherwise provided in this Code section, it shall be unlawful for any person under the age of 18 years to possess or have under such person's control a handgun.