Current federal law sets the minimum age of handguns to 21, but 18-year-olds can buy assault-style weapons like AR15s.  There are also loopholes which allow 18-year-olds to purchase handguns at a gun show or from private collectors. They can also legally receive them as a gift from a parent or guardian.
The minimum age of all gun ownership should be increased to 21 and all loopholes that allow 18-year-olds to obtain weapons should be closed. Teenagers brains are not fully developed and have difficulty in fully evaluating risk. Keeping guns out of the hands of teenagers may go some way towards reducing gun deaths. It could also help prevent school shootings and make schools in the United States a safer place.
It won't stop school shootings or mass killings. The average age of a mass shooter in the United States is 34. The 18-21 demographic is not the biggest culprit in mass shootings so why should they have their gun rights limited? Nor are they the biggest culprits in gun-related homicide cases. 18-21-year-olds commit far fewer firearm offences than the 20-24 demographic. Raising the minimum age of firearm ownership to 21 doesn't make any empirical sense. It is just an arbitrary age introduced by people who want to take our guns away. If anything, taking guns away from the 18-21 demographic leaves one of the most vulnerable segments of the population without protection. They are not the biggest gun offenders but are the biggest victims of gun-related crimes. They are statistically most likely to be a victim of violent crime. This demographic, therefore, needs guns for protection more than any other age group.
[P1] Teenagers brains are not fully developed to evaluate risk [P2] They should not be permitted to own weapons.
[Rejecting P2] The 18-21 demographic is not the most dangerous gun-owning demographic. A rise in the minimum age for gun ownership would be unfair. They are actually the biggest targets of violent crime. Therefore, they need guns as protection more than any other age group.