Ok, can of worms ahead. I just had a bit of a discussion about this with Russ Ouellette, regarding pipe tobacco vs. cigar tobacco aging and processing.
*Generally* speaking, all tobacco mellows and thus improves with age because when it's fresh off the plant it's simply unpalatable. Curing is the first step of the process, which initiates the first massive burst of aerobic fermentation and gassing off of the ammoniates.
Again, *generally* speaking, cigar tobacco is aged a minimum of 2-5 years, through very particular processes. Pipe tobacco is similarly processed. Big producers that can afford to warehouse quantities are also able to extend the aging process for particular leaf--for example, GLP's Cumberland utilizes a mahogany Kentucky with 20 years on it. This does not mean Greg has pallets of tobacco bales out in his shed waiting to be blended; rather it means that his supplier does in the warehouse. Many cigar makers are now touting their closely-guarded aging process as a way to market their product--and rightly so, because it ain't cheap and it works.
Now, after the tobacco is processed, be it cigar or pipe, a lot of things can happen. After curing, the leaf is sort of frozen in time--it won't change much from 5 years to 50 years (theoretically, provided it's kept in a hermetically sealed chamber under zero pressure); that is, until it's
A) exposed to moisture
B) blended with other tobaccos
C) blended with additives / flavorings / toppings
D) placed under pressure / heat
Once it's tinned, then another thing happens--it undergoes another short period of aerobic fermentation, followed by extended anaerobic fermentation. That's where the magic happens. In the case of cigars*, after rolling there will be mainly aerobic fermentation that drops quickly to a minuscule amount, and thus it takes a good deal of time longer for changes in taste to be noted. (*Because most cigars are not sealed. In the case of cigars that are tubed, the tobacco behaves the same as pipe tobacco in a tin.)
As a *general* rule of thumb, Virginias change with age more noticeably because of a higher sugar content. A lower sugar content means that there's just less for the microbes to feed on. But all tobacco will age, and most will "improve" to the extent that they become less chemically astringent--there's your mellowness. The magic happens when the different flavors of varying component leaves age together in a sealed environment; there's a Japanese term for art that is created by chance and serendipitously discovered which, for the life of me, I can't think of at the moment. Those who know the language, please feel free to chime in. Anyway, that's what happens.
Also, again as a *general* rule of thumb, the milestones for Virginia aging (in the tin, that is) are at a year, four to five years, then around ten. After that, it's hard to chart--pure Virginias can go indefinitely, while other components like Latakia or burley may fade into tastelessness. Greg Pease has a great blog article about his experiences, and that guy has had a lot of "vintage" tobaccos. From my personal experience (and a few others like DocWatson who were at the Kaywoodie event this past December), Cope's Escudo becomes a religious experience after 60 years. In fact, I believe it can even be elevated to sainthood (Catholics, feel free to chime in on that one). On the cigar side, I've got some Belindas that have sat in the humidor for ~22 years that are frickin' divine as well.
Now, with all that being said, I don't think that as a rule "vintage" tobacco is necessarily all that much better than "fresh" tobacco. It's ready to smoke and be enjoyed when it comes to you, and that's how I take most of it. There's also the price point of buying vintage, either from a well-stocked humidor or on an auction site--for the most part, the math doesn't work out. The main point to take away--cellar what you like, enjoy it as it ages, and don't go crazy looking for the holy grail of ancient tobacco.
That is, unless you're inclined to.
