I am familiar enough with the local state law that a juvenile "felony of violence" is a state level disqualifier, but a juvenile non-violent felony is not.
However, I did not know how things worked at the federal level.
Every state is different. The federal government does not even prohibit juveniles that serve lengthy sentences from federal charges. Juveniles that go through the federal system with federal felonies are not even treated as felons once they are released.
Juvenile courts in many states are a joke, so it is probably a good thing they don't often count the same. In many states juveniles cannot even get jury trials (Constitutional requirement), and if charged with a crime a "conviction" or finding of delinquency/adjudication is almost certain in some places.
You have not said what state you are from, which although you are asking for the federal law, makes all the difference.
The federal prohibition exists based on how things are dealt with at the state level.
In some states juveniles can never be convicted. In others they can be convicted.
In some they can expunge thier record completely, with the court essentially removing that the conviction ever happened. In others expungement is not an option.
In yet others the term "expunge" merely means sealing of the records to various extents and may or may not remove disqualification, but the crime is still considered to have legally happened, record access is just reduced to certain people. In others expunge may remove the finding of guilt, but the record remains. The term varies from state to state. In those that use the term to refer to actually removing the finding or erasing the record altogether it undoes the conviction because it legally never happened, and the crime is federally as if it never happened. In those where it merely becomes sealed and harder for people to access, it may be treated differently if the crime is a "felony conviction", because then it still legally happened.
Further complicating matters is that the individual in question is subject to the laws and restrictions that were in place at the time of thier court proceedings. Not necessarily current standards. So if for example the state decided to become more restrictive of minors after committing offenses in the 90s, but the offense happened in the 80s, then the 80s statutes would normally apply. So just looking up current laws does not tell you what laws, restrictions, and sentencing guidelines existed at the time.
Retroactively applying new laws or guidelines is generally not done and may be against either state or federal law in different scenarios.
