You might need to talk to a lawyer about this.
A friend had a juvenile diversion sentence for something in CA that was a misdemeanor in the 1980's when he was charged and diverted. CA subsequently has made that offense a felony, and started entering stuff into some shared database of offenders. So he owns NFA and many Title I guns....and all of the sudden he is denied on a NICS check, they say he is a felon.
He hired a decent attorney that is RKI on the gun stuff, and took his guns to an FFL for "repair" in the interim while it was straightened out.http://www.mbassociateslaw.com/
That is the guy he used.
Might be that you can just get that juvenile record sealed and be good to go.....but I am strictly guessing about that.