AT AN AGE when most people are looking forward to retirement, Myron Nightingale, the veteran winemaker for Beringer Vineyards, is producing not only the finest wines of his long career, but also the finest wines the Beringer Winery may have ever released in its 103-year history.

Beringer, one of Napa's most beautiful wineries, has had a long history of winemaking. The original winery was built in 1878 by Jacob and Frederick Beringer. Yet for all of its historic significance, Beringer's name has never been synonymous with the top echelon of Napa Valley's finest wines.

Things began to change when Nestle', the Swiss conglomerate, purchased Beringer, and hired Myron Nightingale as master winemaker in 1971. Until then, Nightingale and his wife Alice had worked at the huge Cresta Blanca Winery in Livermore, Calif., where they had achieved a modest degree of fame for a rich dessert wine called premier semillon, made from spraying semillon grapes in the winery with spores of Botrytis cinerea. Botrytis cinerea is the mold that normally attacks grapes such as riesling, sauvignon blanc and semillon under certain climatic conditions in the fall. The result is often a highly concentrated, rich, complex, majestic dessert wine that can age for a decade or more. The Nightingales were the first to produce a botryised wine in America in 1956 while at the Cresta Blanca Winery.

Myron Nightingale arrived at Beringer to become the beneficiary of a revival of the winery and a new commitment by the owners for high-quality wine. Extensive restorations were made with modern vinification equipment purchased and new vineyards sought out and bought.

Even after Nightingale's arrival to Beringer, the Beringer wines continued to fall within an acceptable range of quality--good, but not terribly exciting. However, starting in 1978, the wines have gone from one strength to another, frequently representing some of the best examples of their type, but also the double virtue of being reasonably priced.

Beringer, like many California wineries, offers some pricey, top-of-the-line "Reserve" wines, in this case a chardonnay and a cabernet sauvignon. Yet unlike many California wineries, the "Reserve" designation is not used in a given vintage if the quality of the wine does not merit special status. Consequently, there is no "Reserve" 1979 Cabernet Sauvignon from Beringer.

Additionally, Beringer's "Reserve" wines, while expensive at prices of $14 to $16 a bottle, still sell considerably below the outrageously priced, widely known "Reserves" from other highly regarded California wineries. Robert Mondavi now is offering the "Reserve" cabernet sauvignon at a staggering price of $40 a bottle; Sterling Vineyards continues to charge $25 to $30 a bottle for its "Reserve" cabernet sauvignon, despite the wine's record of mediocrity over recent vintages.

Beringer's 1978 "Reserve" Cabernet Sauvignon displays a lovely, black-currant, spicy fruitiness, medium to full body and moderate tannin. It is a delicious wine made for drinking now, yet offers the hope for further improvement if cellared for another two to four years. The Beringer "Private Reserve" 1980 Chardonnay is typically Californian with big, bold, aggressive, oaky, spicy, lush intense vanilla and oaky flavors, tremendous extract and a long lingering finish. It is a full bodied wine that is best consumed over the next two years.

While the two "Reserve" offerings are highly sought after by California wine connoisseurs, Beringer's other wines are quite fine and generally available in good quantities at very moderate prices.

The 1981 Chardonnay from the Gamble Ranch ($10.99) is not dissimilar from the 1980 "Private Reserve" Chardonnay. It is a big, spicy, richly textured wine with plenty of vanilla oakiness to match its rich, viscous butterscotch flavors. The 1981 regular Chardonnay ($7.99 to $8.49) is made in a very clean, fresh, fruity style, which does not have the oakiness of the winery's two more expensive chardonnays. It is an ideal quaffing chardonnay which shows very good balance, flavor and delightfully crisp acidity.

Beringer's success with the white wines is not limited to merely chardonnay. The winery's 1981 sauvignon blanc called fume' blanc ($7.99) has much of the style of a big, slightly herbaceous, but smoky, earthy, pouilly fume' from France. It is a splendid choice with shellfish, especially mussels. Beringer's three least-expensive white wines include two well-made varietals, a 1981 French Colombard ($4.49), and a 1981 Chenin Blanc ($3.99), as well as an extremely good generic, a 1981 Chablis ($2.99). While the French Colombard is remarkably fresh, crisp, steely, with a very nice aromatic bouquet and dry finish, the chenin blanc is a contrast, offering fruity, slightly sweet soft flavors in a light- to medium-bodied format. Both wines were bottled as quickly as possible after the vintage to preserve the inherent freshness and fruity character of the grape. The French Colombard is, however, given a brief kiss of oak aging to give it an extra dimension of flavor interest. The generic 1981 Chablis is a very fine value in inexpensive white table wine and shows surprising fruit and character for a wine of its class.

Despite these obviously successful white wines from Beringer, the wine that the Nightingales are most excited about is the botrytis-induced 1980 Semillon, which is an unctuous, rich, very intense dessert wine that the Nightingales first produced in 1956 at Cresta Blanca. It will be called simply "Nightingale" and will unfortunately be available only at the winery at a price that has not yet been determined. Having tasted this wine, I can recommend to those wine enthusiasts who will be visiting Napa Valley this summer, to make the trip to Beringer to latch on to a bottle of this incredibly rich, luscious dessert wine, which has uncanny resemblance to a great French sauternes.

On the red-wine ledger, in addition to the quality 1978 "Reserve" Cabernet Sauvignon, Beringer's regular cabernet sauvignon, a 1978 ($6.99 to $7.99) offers less complexity and depth, but is grapey, supple and easygoing; it will have many a friend in wine-drinking circles.

Beringer Vineyards merits consumer attention, not only because of an obvious commitment to quality, but also because of a realistic pricing policy that makes Beringer wines score high on any price/quality scale of performance.