The age restriction is not reasonable.
Citizens are either adult at 18 or they are not. If they are not, they cannot be criminally charged as an adult; They can not contract; They can not marry; They can not be sent into a combat zone; They can not be drafted, etc. And we would have to change the Constitution to take away voting rights - The Court cannot do this.
If this age group are adults, there are no compelling arguments to maintain a handgun prohibition when allowing other, more powerful firearms (meaning the law is not narrowly tailored to fit the reasons). Remember, the federal law does not prohibit the ownership, it just prohibits an FFL from selling a specific firearm to anyone in this age bracket. The Heller Court has already listed the prohibited item as the quintessential tool, preferred by the public, for self-defense.
The only arguments against, are arguments of emotion, not fact.