In my country, no firearm can be issued to any civilian (certainly not a minor), without verification and license from police.
In Texas, there is no minimum age for purchasing ammunition beyond federal limits, no requirement for an ammunition seller to keep a record of the purchaser, and no specific license to buy or sell ammunition, according to the Giffords Law Center.
https://www.kxan.com/investigations/uvalde-shooter-had-1600-...
Salvador Ramos, the Uvalde school shooter, legally purchased two AR platform rifles Ramos got his guns legally through Oasis Outback, a Uvalde sporting goods store and federal firearms licensee, according to published reports. He also purchased hundreds of rounds of ammunition, on his 18th birthday.
https://edition.cnn.com/2022/05/25/us/uvalde-texas-school-sh...
I know the USA has a bad habit of buying things on credit, but firearms & ammo should never be allowed to be purchased on credit. Let it be purchased only after a verification and license from police, and only via debit card or bank transaction with proper legal paper trail, not credit or cash. And any firearm and ammo purchase should be ratified with local police, so they know if someone is making a suspicious purchase.