This is the only thing bugging me too. Still, I am a paralegal and know that the case #'s, Defendant #'s and names, DOB and date of crime are enough to prove to me that this is indeed the same person.
I don't work much in criminal law anymore and am not familiar with FL statutes on this kind of thing BUT...what I suspect here is this -
DS was technically 20 at the time of the crime - still of age and considered an adult. However, when you have a gang type attack with several defendants like this, it is common to plea many of them down. In other words, I am thinking there was some deal struck to treat him as a minor, instead of trying him as an adult. If that was the case, there was probably an agreement to have his record sealed....although that too might have been conditional. IOW, if he got in anymore trouble at all, all bets were off and the record would become public. There are all kind of deals that can be struck and the "conditions" are endless.
This is purely speculation on my part. Obviously I am not sure at all that is what happened, but it is the only thing that makes sense to me. It would also explain how he might have slipped through the background check re: enlisting in the Marines.