It should be against the law for 16 yr olds to own or have a gun to rob with...
Robbing is illegal, robbing with a gun is VERY illegal, 16 year olds can have guns as far as I'm concerned, as we rely on parents to do their part first, the parent has to buy them the gun anyhow (at least in states that allow minors to possess firearms).
However, if the 16 year old breaks the law with the firearm then the PARENT should be held accountable. Actually the parent should be held accountable if their child breaks any serious laws...