There are various degrees of background checks. The military has the opportunity to dig deep. Juvenile records are not available on all checks. Your best solution right now is to hire an attorney in your state who is familiar with criminal charges and background records to help you do as much as possible to protect your background information.
Whether an employer chooses to hire you is up to the employer. This is not really a criminal law question.
Since you were a juvenile when this occured and if you have never been convicted of any other charge since the employer should not hold this against you.
If it has been over ten years since you finished your sentence, and you have no other arrests, you MAY be eligible to seal. You should speak with an attorney in person
I agree that it's possible that some agencies will know about your juvenile history and others will not. The Florida Statutes on expunction only allow certain agencies information on whether something was removed from a record and also whether something is sealed on it. If you have no other adjudicated history and you weren't adjudicated delinquent on the offense you're speaking of, you may be eligible to seal. Without more information, no one can tell you. You should look to hire an attorney to review it for you if you think it's causing you problems.