In an age when humanity lands a spacecraft on a comet and reconstructs the genomes of long-extinct animals, it may seem ludicrous to be perplexed by a glass of wine. Yet these small moments of bewilderment are integral to the best wines, which, like all that is beautiful, rise above definition.

Chief among wine’s mysteries are how and why it ages. By aging, I don’t mean simply enduring over years, youthful fruit and fragrances intact but relatively unchanged. I mean evolving, from a primary state of freshness and rough vitality to vigorous middle age, when grace, complexities and nuances tantalize and enthrall, through a long period of gentle decline and resolution, which offers its own subtleties and allure until, gradually and finally, the wine flickers into opacity.

Why is it that some wines age and others do not? What conditions encourage aging? And what happens to a wine as it ages? We do know that wines age best when stored in cool, dark, humid, tranquil conditions. But there is a lot that we don’t know, and much of what we assume to be true in fact is not.

Most wine authorities insist, for example, that Beaujolais and fino sherry must be drunk young and fresh because they cannot age. What these doggedly repeated platitudes really mean, however, is that mass-produced Beaujolais and fino sherry cannot age. When made with fine materials handled with meticulous care, these wines evolve beautifully.

Here at Wine School, we refuse to be bound by clichés and bromides. Rather than bone up with textbooks as if wine were some sort of junior-high requirement, we go right to the primary source material, the wine itself. Each month, we focus on a particular type of wine, drinking it attentively in a relaxed, natural setting, with food, family or friends. The aim is to achieve a clearer, more confident sense of our own tastes by carefully noting reactions over the course of a meal. Drinking, rather than tasting, offers the opportunity to meet a good wine on its own terms, to see the entire movie play out rather than settle for a single still shot.

Aging was foremost in mind this month as we examined Rioja, a wine that can age particularly well. Producers who make Rioja in its most traditional form take on the responsibility of aging wines themselves before releasing them. This frees consumers of the most serious obstacles to enjoying aged wines: the expense both of investing in wines that will not be consumed for years and of storing those wines under ideal conditions.

Why would producers do this themselves? It’s partly a matter of style and tradition. For years, the best Riojas received prolonged aging in barrels of American oak before further aging in bottles. Today, many Rioja producers have abandoned this approach to emphasize other qualities in their wines. But a handful continue to make wines by this traditional method.

The labeling of Rioja was likewise organized by how the wines were aged. Wines made for immediate consumption were called joven, or young. Red Riojas that received a modest amount of aging before release (two years’ minimum is the current requirement) were called crianza. Reservas required a minimum of three years of aging, and gran reservas needed at least five years. Many producers of more modern styles of Rioja have abandoned these designations, while the most traditional producers may age their wines far longer than required.

Good gran reservas can be sublime, but they can also be hard to find and more expensive than we prefer at Wine School. So we picked a middle ground and chose three Rioja reservas: the 2005 Viña Ardanza from La Rioja Alta, the 2003 Viña Bosconia from R. López de Heredia and the 2009 from Muga.

The age variance clearly stands out among these wines. At 5 years of age, the Muga showed itself as the young’un in the group. Its berry flavors, if not flamboyant, were still brash and somewhat jagged, the tannins still roughly apparent. With a good, fatty leg of lamb, the wine was delicious, though its aromas and flavors remain clearly primary, still redolent of the spicy red fruit that is characteristic of young Rioja. Nonetheless, the wine has the body and richness to evolve further. If it does not yet offer the pleasures of an aged wine, it will with time. Check back in five years, then again in 10.

As a producer, Muga sort of splits the difference. It makes traditionally styled wines like this reserva but does not age them as long before release as the most classic producers, like López de Heredia and La Rioja Alta, which made our other two wines. Muga has just released its 2010 reserva, for example, while the ’05 Ardanza and the ’03 Bosconia remain the current releases from our other producers.

The Ardanza, at 9 years of age, has clearly begun its transition toward an aged wine. The years have buffed its ripe berry and earthy mineral flavors, while the soft vanilla of the American oak serves as a velvety frame, cushioning the wine. The aging shows in the smooth integration of the wine’s various components, yet a sharp tingle of fruit and acidity remains, indicating that it still has quite a few years before it. Gran reservas can age for many decades, but what about reservas? Hard to answer, but I recently drank a 1974 Contino Reserva that was absolutely delicious.

The last wine, the ’03 Bosconia, was elegant, sleek and subtle, showing great finesse in its graceful, rich yet burnished flavors. This is a well resolved, fully revealed wine, further along its trajectory than the Ardanza, more than the 2 years of age difference would suggest, most likely because of the peculiarities of the very hot 2003 vintage. The Bosconia from 2004, a much better vintage, will arrive soon, and my guess is the wine will last far longer than the ’03.

The pleasures of aged Riojas were not lost on readers. “Rioja is probably the only region where you could find top wines selling at this price range,” noted Adrian Wu of Hong Kong, who also lamented the dwindling number of producers who are still doing the work of aging.

Ali of New York, who found the ’05 Ardanza, wrote online that the wine was firm at first but became “richer, lovelier, fresher” with time and food. And Seancpa of Pleasant Mount, Pennsylvania, who found an Ardanza from 2001, an excellent vintage, found the wine light on the palate, with less immediate fruit flavors than a younger wine but with more depth.

“The word that comes to mind about aged Rioja is smooth,” Ali wrote, and I agree. Wines like these are to be treasured. As the taste for wine has become more widespread and democratic, a smaller percentage of its audience can afford to invest in aging wines, so it is a treat to find affordable wines that are pre-aged.

This is not to say that aged wines are necessarily better than fresher, younger wines. Even with wines with the potential to age, some people prefer them young. There is no universal moment when a wine is at its peak. It depends on your own taste. The difficult trick is to build up reference points so you can make up your own mind.