>"must be 21 to purchase a firearm"
It depends on what type of gun and what type of transaction. Under federal law when dealing with a Federal Firearms Licensee, also known as an FFL, you have to be at least 18 years old to purchase a long gun, and at least 21 years old to purchase a handgun. Here in Texas to purchase a long gun the rules are the same. But unlike federal law, Texans can purchase handguns at 18. …
https://www.uslawshield.com/buying-a-gun-in-texas/