Setting the legal age of procurement at 21 won't stop school shootings or mass killings. The average age of a mass shooter in the United States is 34.
As the 18-21 demographic is not the biggest culprit in mass shootings, there is no reason to limit their gun rights.
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 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 and violent crime more generally. This demographic needs guns for protection more than any other age group.