I’m very wary of this idea, mainly because the juvenile justice system has lesser due process protections than the criminal court system, such as the right to trial by jury. It was designed this way because the juvenile system is not supposed to operate in the traditional realm of crime and punishment, but as a means of rehabilitating a juvenile who had been adjudicated delinquent. At least that’s the theory. That’s why juvenile adjudications are treated differently than criminal convictions in the regular court system.
In most states the option exists to try juveniles as adults for serious, violent offenses. My preference would be that a juvenile be tried as an adult in these types of cases, which would remove his Second Amendment rights upon conviction. By applying a life-long civil rights disability to a juvenile, it defeats the point of the juvenile justice system. Presumably, much like Lautenberg, there would be an ex post facto component that would apply this retroactively, creating thousands of newly minted, and blissfully unaware felons overnight.