The magic of Vintage Port is that it is so different in every phase: as a young wine it's all powerful and youthful fruit characters, the effects of spirit maturation and integration are revealed; after decades really fine mature old spirit dominates the wine. Vintage Port is amazing in all three phases.
Most VP need upwards of fifteen years before they are ready, and may last for decades more. It is important to note that VP suffers a flat spot about 5 years from harvest, and unlike other drinks which suffer similarly (Riesling, for one), the ‘dip’ period can be 10 years or more of surly adolescence. Drink them less than 5 or more than 15 years from harvest is a simple rule of thumb to observe.