The Adjudication as a delinquent has no bearing at all in this context, no matter if it was withheld or not.
So to answer your question NO, you can not ever legally own a gun. This will hold especially true if you apply for reprieve for prior convictions that are firearms based. Since you were convicted of theft of a firearm (Again, age does not matter) you will not get that overturned to allow you to own a gun. Furthermore, if you do end up with a firearm and are caught with it, and the investigation into your background shows this conviction, you will get charged with the federal laws pertaining to "Possession of a firearm by a convicted felon".