Aging doesn't stop when you open the tin, but it definitely changes. It's actually not all that mysterious, but it is quite complex. "Aging" actually covers a multitude of things. There are both biological metabolism and chemical reactions taking place. In the case of fermentation, both bacteria and yeasts are actively doing their thing, and there are organic reactions that will change in response to the environmental factors within the tin. Even things like storage temperature can result in dramatic differences between two identical tins of tobacco over a period of years.
Attempting to accelerate the process won't work. Prematurely depleting O2 from the environment won't work. Basically *anything* that changes the environment will alter the outcome. Sure, you'll get *something* that looks like aging, but it won't be the same as aging that takes place naturally.
There are aerobes that requite O2, facultative anaerobes that continue to metabolize, though through different pathways, with or without the presence of O2, and there are obligate anaerobes that go dormant in the presence of O2. These can be yeasts or bacteria, each doing their own thing. Remove the O2 too early, and aerobes won't have their chance to dance. Keep introducing O2 through opening and closing the jar, and obligate anaerobes will stay in bed. Some bugs are pretty sensitive to temperature, so heating a tin, or cooling it off dramatically will drastically change what's going on in that little biosphere.
Complicating this is that there are cascades of organic reactions taking place - production of alcohols, aldehydes and esters, fermentation of acids. Different reactions will have different rates depending on the ever-changing environment (what's available) and temperature. When I say it's a complex system, I ain't kidding. Wine makers know how important environmental factors are. Cellar temperature, cork porosity, even *gasp* screw caps change the way a wine ages once it's bottled. Pipe tobacco's not much different. Two bottles of the same wine aged for a decade or two can turn into nail-polish remover (not really) in one environment, or evolve into something spectacular in another. I've not had tobacco turn noxious from poor storage, but I've seen very different results from a few years under different storage conditions.
As one thing happens, it sets the stage for another. Change something early in the game (evacuating O2), and what you'll find years later will be surprisingly different. Butterflies and hurricanes.
As you might figure, I've done considerable messing about with this stuff. To those who call "Horsehit," with nothing more than opinion to back it up, I offer the following experiment. Take any natural tobacco that you like, from recent production. Divide it into four clean canning jars so that they are all quite full, but not tightly packed. Put three of the jars away in a cool, dark closet. The fourth, you will open and re-close daily, as though you're smoking the tobacco. At the end of a year, compare the regularly opened jar with one of the sealed ones. You'll notice a distinct difference. Re-seal the year-old jar, and put it away. Continue for another year, and then examine the difference between the regularly opened jar, the first you opened, and a third one, opened for the first time. Three different results - one tobacco.
I can't claim to even begin to understand what's going on, and have only scratched at the surface. But, it doesn't take much looking to realize that what's going on is surprisingly complex.