It's based on timeline I came across
78 years ago
30 years ago (Garp would be 48 so I am sure Dragon was already in twenties by this time)
- Kuzan joined (19 years old approx)
- Robin was born
24 years ago - Roger execution
Dragon looked like this. Young but could easily be in his late twenties
22 years ago
- Ohara incident.
- Kuzan and Akainu were already Vice Admirals and kuzan would be around the age of 27.
So if we assume Dragon is slightly older than kuzan then him being Admiral isn't far-fetched.