So basically, nothing is stopping Dragon from becoming an Admiral at thirty or younger.
Dragon may or may not have been an Admiral, but he definitely had a superior relationship to Akainu than just rival or friend, it's clear that Akainu is hyper obsessed with Dragon, not the other way around.
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Straw Hats only know and reveal information that Oda finds convenient and wants revealed, I mean we only found out that Dragon was even a Marine like in the last hundred chapters lol, even though Marines like Akainu and Sengoku absolutely would've known and could've revealed it during Marineford and just didn't.
Also, Dragon could've used an alias while a Marine, there are many possible explanations that don't create holes in Oda's writing.