You need to seal your juvenile record if you can, whether you join the Marines or not. You need to disclose any juvenile history to the Marines. Even though sealed, it can be unsealed. Most likely your juvenile record will not affect you getting into the Marines. If you disclose, you will never have to worry that someone will find out. When you disclose, note that it was seal (provided that it was sealed).
The above answer does not create an attorney-client relationship. The information provided is general to anyone, and is not intended to be advice to anyone's specific situation. You should consult an attorney immediately concerning your legal issue. The information provided is not intended to be relied upon as a final or conclusive solution to your situation.
Keep in mind that a factor for enlistment will be the type of offense you had. For example if it was drugs or a sexual assault that will be difficult to overcome.
At enlistment that do a NAC, a National Agency and local records check. That will usually show arrests and convictions. They will use that check to determine your eligibility for enlistment.
While I agree with my friend Mr. Cave, let me add this, having just gone through this exact process with someone enlisting in the Army. The first thing that you need to do is to find out IF it is the FBI's computer data base, the NCIC. Different states have differing rules for how (and when) they report juvenile records, so it may or may not be in the NCIC system. Regardless, on the enlistment forms they ask you specifically about any juvenile arrests or convictions. Lying on that form is both a federal and military offense, which not only could get you discharged, but also court-martialed. The Recruiters btw are not lawyers, so do NOT rely upon them for legal advice.
Additionally as Mr. Cave noted, much will depend on just what the conviction was for, how long ago it was, and what the sentence was. For example, if you are still on any type of probation or court-monitoring, you are ineligible to enlist while in such a status.
What you need to do is to find out first of all, is to find out IF the FBI has you in their record-keeping system. Check out and follow this link to do so:
If they are, then they will be available to the military on ANY background check. However (and this is generally the problem area), if the Judge in your case ordered your records sealed, that may have happened locally, but not necessarily communicated to the FBI. If the FBI's records do not reflect that they are sealed, there is a process to "amend" your FBI records which the link will assist you with. My advice however, is to first go to the lawyer that represented you in the juvenile matters and see if she/he knows or has a copy of the Judicial Sealing Order if they can get you one to submit to the FBI if they do refect your record.