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this is the Draconomicon version I posted.

I feel it's not a measure of years, it's a measure of towns eaten and magic wrought. If they're a fairly active dragon getting lots of exercise, fresh virgins, and artifact level loot then they could be progressing an age category every few centuries or even a few decades if they raze a good sized empire or other activity. On the other hand, if the mostly sit in their lairs, count their hoards, and eat only the occasional mountain goat or hapless orc then they may never achieve even their first virtual age category.
A good analogy would be a retiree. There are those retirees who just slowly sink into a reclining chair after they retire, they never really do much or keep themselves up. These are the people who tend to age quicker since they aren't keeping healthy and in this analogy are dragons who never do much after Great Wyrm. These are the dragons that after conquering a dwarven mine they just sit on everything until a bunch of meddling dwarves and their halfling too show up and ruin their nap centuries long.
On the other hand, there are those retirees who decide that they've spent their whole life preparing for retirement and now they plan to do EVERYTHING. They vacation, they walk, they hike, they take up kickboxing, and they never let themselves get decrepit even if they get old. As dragons, these are the types of people who continue to progress virtual age categories past what "super dragon puberty" does for them.