Provided blends are cellared in decent conditions, they should 'improve' with age for at least one to two decades and in all likelihood for longer. When they 'peak' is probably as much dependent upon individual taste and storage conditions as the blend itself, but I suspect that in most all cases if reasonably well stored it is sometime after at least a decade. I also suspect that the greatest improvement comes during the first decade, that for the most part the peak is sometime during the third decade, but that the most memorable experiences will come, together with some disappointments, from well stored tins closer to a half century old.
John C. Loring, 2000-05-04
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I've smoked Three Nuns that was over 40 years old, and was magnificent. I've also smoked Balkan Sobranie of a similar vintage, and found it over the hill, though a 30 year old tin, sampled the same night, was spectacular. [...] Any truly exquisite tobacco will stand up to the test of time, and be bettered by patience. When it will begin to decline is highly speculative. Five years is safe, as, probably is ten. Beyond twenty years lies the gray zone.
GL Pease, 2000-05-03
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There is some practical upper limit to this, just like with wines. At some point, the tobacco's smoking quality will actually begin to decline. This turning point is determined by several factors, including the types of tobaccos in the blend, the conditions under which it is stored, how much air was left in the tin at packaging time, how moist the tobacco was originally, and so forth. As a general rule, the darker a tobacco, the longer it will age, while lighter, more delicate leaf loses some if its flavour and aroma if aged too long. (Interestingly, some light tobaccos will get quite dark after a few years of aging. Lemon Virginias, for instance, can turn to a lovely nut-brown.)
GL Pease, 1997-10-13