https://static.prisonpolicy.org/scans/bjs/jdfcjs.pdf
Federal Juvenile Delinquency Act
An act of juvenile delinquency is a violation
of Federal law committed by a
person prior to age 18 which would
have been a crime if committed by an
adult (18 U.S.C. § 5031). Under Federal
law, a person accused of an act
of juvenile delinquency may be processed
as a juvenile provided the
person has not attained age 21.
Adjudication of juveniles in the Federal
system is limited. Federal law requires
that prosecutors restrict proceedings
against juveniles to those cases in
which they certify to the court that ...
...
the offense charged is a violent
felony, a drug trafficking or importation
offense, or a firearms offense (18
U.S.C. § 5032).
It looks like a pretty big prosecutorial discretion / prosecutorial limitation
is in place.
If the illegal applies while still under 18, the feds can't prosecute them
as an adult, and they can't prosecute them as a juvenile. And
they only have 90 days to get an indictment to charge them as an adult.