The FBI will not seal the interstate record and would expunge it only upon proof that Massachusetts did so, which as you've been told will not occur in these circumstances (or virtually any other). Your only recourse is to try to reopen the juvenile case and get the delinquency removed due to some defect in the proceeding.
You cannot expunge a juvenile record in Massachusetts, but you may be able to have your juvenile record sealed if the cases have been closed for at least 3 years. However, if you are applying for a federal law enforcement job, they likely will still be alerted to and/or be able to see your sealed record. Here is a link to the Petition to Seal that you will need to fill out in order to have your juvenile record sealed: http://www.mass.gov/courts/docs/probation/sealingpetition.pdf
If you want to seal your juvenile record, you must petition the juvenile court in the State of MA in the jurisdiction where you had your charges. Since you are 25 years old, you would qualify. It is my understanding that juvenile records cannot be expunged in MA. Expungement means to completely remove the records from the criminal records file and that process is not available for juveniles in MA.