This just dawned on me recently and it is that Oda may be using the history or piracy as another reason for the Great Pirate Age.
Now I know the age was 'started' by Roger but I highly doubt on that instant everyone found crews and went off to sea, it was more likely that they were waiting around for something to do and then he gives them something and all the pre-formed crews head off to destiny.
the reason I believe this is because the Great Pirate age in the Caribbean back in the past, was formed after the wars the country's of (Britian, French, Spanish etc etc), waged on each other, and after the conflict was over the soilders from those wars were left with nothing to do to support a way of life so they decided to make they're own on the high seas.
Now we know there was a nation that used to exist but now doesn't, my theory is that all of the pirates or at least most of pirates are decendants of the soldiers that fought in that War (which I believed happened) when the WG gained control of the world in the Void century.