Merchants and critics attach drinking windows to age-worthy wines, and buyers treat them as though they were measured facts. They are structured estimates, and understanding how they are built explains why they are frequently wrong.
Structure is the primary input
Tannin, acidity and to a lesser extent alcohol are the components that preserve a wine, and their quantity sets the outer limit of its life.
A wine with abundant firm tannin and high acidity has the raw material to develop for decades. One with little of either does not, regardless of its price.
This is why structure rather than fruit intensity is what tasters assess when judging ageing potential in a young wine.
Vintage conditions modify the estimate
Growing seasons produce wines with characteristically different structures, and a region's cool years and warm years age on different schedules.
Warm vintages often produce wines that are approachable early and mature sooner, while cooler ones can be austere young and last much longer.
Assessors who have followed a region for years use previous vintages of similar character as the reference, which is essentially pattern matching.
Producer track record does the rest
Two wines from the same appellation and vintage can have very different lifespans depending on how they were made.
Extraction, oak handling, filtration and sulphur levels all affect longevity, and none of it appears on the label.
An estimate for a producer with a long history rests on how their older vintages actually behaved, which is the most reliable evidence available.
Why windows are usually too conservative at both ends
Windows are published as a service to buyers, so they tend to open once a wine is broadly enjoyable rather than at its earliest point of interest.
They close before the wine is finished, because being wrong in that direction disappoints fewer people than the reverse.
Bottle variation widens the true range further still, since two bottles from the same case can differ noticeably after twenty years.
Reading a window as a curve
A wine does not switch on and off at the stated dates, and treating the window as a plateau with slopes at either end is closer to reality.
Opening bottles across the range is the only way to find where a particular case sits, which is one practical argument for buying more than one.
The information gained applies only to that cellar, since the window depends on how the bottles were kept as much as on what they contain.