This is a tricky subject, and I don't want to pretend that I have the experience necessary to consider myself an expert. Let me try nevertheless to say a few words about this issue. The crude message is this: while the mass produced, cheaper German wines are made for instant consumption, its fine Rieslings can demand considerable patience.
How well a wine ages is determined by a large number of factors, which can be difficult to assess. The factors at work include the grape variety, vintage, concentration, acidity, residual sugar, botrytis, tannin, and storage conditions. There is also the matter of personal taste, of course. Some prefer the youthful fruity aromas over the sometimes slightly weird signs of maturity, such as the notorious "kerosene" smell of some older Rieslings. When a wine tastes good, why wait and take any risks? I find this is often the right attitude, especially for dry wines. Many wine lovers will argue though that the balance and complexity of a good, mature Riesling makes it superior to a young one. If it was a sweet one, its sweetness will be less obvious, reaching a more harmonious integration with the acids. The nose and palate will be more complex, with mineral elements coming more into prominence beside the fruity and floral characteristics, which in the better cases should still be present.
Fine Riesling can be extremely long lived. The more concentrated, in particular the higher in acidity and residual sugar, the longer it will usually improve. In the extreme cases, like TBAs and Eiswein, that can mean decades. Under 10 years of age one should not expect them to reach their peak. 20 years is more appropriate, but few can wait that long. Lower predicates can be approached younger. Many Kabinetts and lighter Spätlesen are quite delicious right from the start. It is widely agreed that as with some red wines, after a first youthful bloom of 1-2 years, some German Rieslings (esp. MSR) close down somewhat, even developing musty odours, only to emerge with renewed vigor and added complexity some years later. Depending on the various factors this can mean two to five years of patience to be exercised. Non-dry Kabinetts tend to have reached their best before the age of 10 years. As the predicates go up, so does this number. "Trocken" wines generally mature quicker than sweeter ones, and do not last as long (rarely beyond 5 years). Exeptions exists though.
