Bacteria do their thing until they run out of food (lactose, etc.) - usually within 60 days. THAT'S why the Federal 60 day rule applies to raw milk cheeses. There's nothing left for bad organisms to feed on.
The bacteria themself start to die off and release proteolytic enzymes into the cheese. These enzymes can have a LOT to do with the final taste AND texture. The longer some cheeses sit, the more these enzymes can work their magic. Same thing happens with an aged wine and to a lesser extent, beer. The bacteria are shot, they have generated CO2 for carbonation and then the enzymes take over. Proteolysis chemistry is really complex, and interesting, but that's the basics of aging. Do cheesemakers need to study proteolytic enzymes? Heck no. Just be patient. It will happen naturally. BUT... that's also why following recipes accurately and consistent technique are important. Everything we do along the way - pH, moisture, lactose content, aging temp, aging time, etc. - creates the correct environment for these enzymes to work.
In the case of Parmesan and other "Italian" cheeses, we add Lipase (an enzyme) in the beginning, so that classic Parmesan taste and smell comes from early enzyme activity, not just bacterial action. I understand that the commercial guys have other enzymes that enhance flavor and speed up the aging process, etc. , that we don't have access to. I'd love to know more about them.
So there is a LOT of difference between a 20 day old cheese and one that's 90 or more.