Question: I had a felony conviction as a juvenile. Am I still eligible to join?
The answer is "maybe." A criminal record is not automatically disqualifying. You will probably be rejected by the military if the crime involved violence or the sale of illegal drugs. Most sex offenses also are disqualifying.
However, some recruits with a juvenile conviction in their pasts have been admitted with a special waiver.
By the way, there's no such thing as a sealed or expunged record, as far as the military is concerned. If you were convicted of a juvenile offense, don't try to hide it, even if the record was officially expunged.
The military requires that such records be revealed on enlistment and in security clearance paperwork. Failure to do so is a felony.
More on criminal offenses, adult and juvenile, that may preclude enlisting or promotion in the military enlistment qualifications.