Again, you would be incorrect. This is almost too easy.
Minimum Age for Gun Sales and Transfers:
Under federal law
Licensed firearms dealers
Handguns
Dealers may not sell or deliver a handgun or ammunition for a handgun to any person the dealer has reasonable cause to believe is under age 21.
Licensed firearms dealers
Long Guns (Rifles and Shotguns)
Dealers may not sell or deliver a long gun, or ammunition for a long gun, to any person the dealer has reasonable cause to believe is under age 18.
Unlicensed persons
Handguns
Unlicensed persons may not sell, deliver or otherwise transfer a handgun or handgun ammunition to any person the transferor has reasonable cause to believe is under age 18, with certain exceptions*.
Unlicensed persons
Long Guns (Rifles and Shotguns)
Unlicensed persons may sell, deliver, or otherwise transfer a long gun or long gun ammunition to a person of any age.
I gave you an example - a jurisdiction where a minor may not legally possess a handgun
A minor is someone below a legal age limit. here we use the Hand Gun age limit as out bench mark for "minordom"
A minor with regard to hand gun ownership can also be someone old enough to live on their own.
If they live on their own they by law may not own a handgun as there is no guardian to consent to legal possession of the handgun they could not have legally acquired.
As such, any discharge of said illegal weapon, even in self defense, would be a crime.
So again - a jurisdiction where a minor may not legally possess a handgun a long gun would be required to legally defend themselves with a firearm.
I have shown you to be incorrect on a number of fronts.
a) a jurisdiction where a minor may not legally possess a handgun
b) a small framed person that would not be able to effectively wield a hand gun