Each year, for the past three years, I have hosted a tasting of seriously mature (some might say geriatric) Cape wines. The age guideline for the event's reds is set at 25 years or older, and for whites, at least 20 years. Contemporary wisdom holds that these wines should long have passed their use-by dates.

Of course, every year there are disappointments. Most of the submissions prised out of the woodwork have only recently been properly stored. Wines from the 1940s and 1950s were made at a time when few homes had cellars, and none had air-conditioning. Even their producers stored them badly, with no real protection from temperature fluctuation and variations in ambient humidity.

The exercise is necessarily a hit-and-miss affair. Sceptics are surprised that anything decent emerges. It is wonderful, even breath-taking, when the number of truly extraordinary wines exceeds our expectations. There is also a palpable sense of relief when we are spared too many truly undrinkable samples.

It doesn't make much sense to produce a list of the best wines from each edition of this event. Firstly, they are largely unobtainable. (Just the same, if there is anyone out there with a large stash of Chateau Libertas from the 1940s and 1950s, or Nederburg and Zonnebloem from the early 1960s, I'd been keen to hear from you.)

Secondly, the fact that many have done well over several vintages and at a couple of tastings doesn't guarantee consistency. The circumstances themselves are simply too variable. Finally, the joys of old wines are not something that all enthusiasts necessarily share.

In one respect however, the tasting has wider applicability: with relatively few producers bottling under their own labels in the pre-1980s, there are patterns which are easily discerned. From these it is possible to conduct something of a forensic examination into when the maturation potential of Cape wine changed - from a long to a sometimes disappointingly short shelf-life - and perhaps even why.

Here the key observation is that it is the wines from before the mid-1960s which seem to have survived best. The extraordinary longevity of these more ancient vintages - all of which were made without any technological knowledge - can really only be attributed to the vineyards.

The late Ronnie Melck - who was Managing Director of SFW (which made - or at least bottled - many of these treasures) believed that "greed" had changed everything. "In pursuit of bigger volumes we were less rigorous about the sites and the yields," he told me. The old bush-vine vineyards which supplied the best grapes were replaced, or lesser vineyards added to the fruit resource.

By the latter part of the 1960s, many of the wines were lighter, more polished, less rustic. In the next decade the introduction of the wine of origin legislation forced producers who had been a little loose in their labelling to use the varieties specified on the packaging. To do so they were compelled to include fruit from very young vineyards, further reducing the ageing potential of the wines.

Five years later, the Cape discovered the joys of new wood vinification, subjecting these less robust grapes to dollops of (often sappy and poorly cured) oak. Then the ongoing battle with vineyard virus saw waves of new plantings, with the younger vines yielding more alcoholic fruit.

In the meantime we have become a nation with a penchant for vinous Lolitas. Aged wines require a less superficial intellectual and aesthetic engagement. They are obviously more fragile. The primary fruit notes which account for much of the appeal of younger wines begin to fade, transformed into tastes and aromas which conceal, rather than reveal, the varieties from which they were made. Sometimes there is a fine line between the enjoyment of an aged beauty and necrophilia. Wine encourages a constant determination of that boundary, perhaps with the promise that after the joys of the flesh, there are still the pleasures of the spirit.

First published in Business Day, 21 April 2009