So like the title suggests, it's about Dragon's past. The theory is that Dragon was an Admiral before he became a revolutionary. This is not so unlikely since Garp probably pushed Dragon to becoming a strong marine, just like he did with Luffy, Ace and Sabo. Some people speculate that he either saw/learned a WG secret/past or that one his loved ones like his wife (Luffy's mother) got killed (being his wife is very unlikely, since Luffy wasn't born yet when he was at Roger's execution). This is what made him leave and start the Revolutionaries. Being a former Admiral would speak about his power and justify the reactions people have when hearing about him. Picture Kuzan creating a revolutionary army and growing stronger in both personal and military power over a long period of time. This would certainly justify the "World's most dangerous criminal" title as well.
About the timeline: He was seen at Roger's execution, so he might've just have left the Marines then. Luffy is 19 post-timeskip so his father can be everywhere between 40-60, let's say 50-55. This would make it that he was around 30 years old when he quit being an Admiral, Roger's execution being 24 years ago post-timeskip. Though he might've not been an Admiral for a very long time.
Someone in the forum mentioned something similar to that it would be cool if Oda shows a flashback of Dragon and under his picture it would says "Marine HQ Admiral Monkey D. Dragon". I also think that it would be pretty awesome. It could be that he was just undercover during the execution of Roger to not draw attention to himself. Then we might see him take of the robe on his way back to HQ and there is an Admiral uniform under. Then it could be that he had already learned about to WG secret/past, but didn't leave immediately. Instead he left after they killed Luffy's mother.
I don't know if I believe it yet (or think that it's possible), but I thought that it was an interesting perspective regarding Dragon's history so I added a little to it.
