I agree with the rule that most Zins should be drunk up within 5-6 years, and that they show best in the first 2-3 years, but I think there are exceptions.
A '78 Lytton Springs Winery Zin drunk in '91 was outstanding. Still rich and well concentrated in the fruit department. And it still tasted like Zin, too, with added layers of complexity, not just like generic old wine, which is where most aged Zins go.
A '91 Ridge Lytton Springs drunk in '00 performed pretty much the same trick as the '78 Lytton Springs.
Several bottles of early '90s Lytton Springs and Ridge Geyserville have held up nicely and been quite enjoyable at 7-10 years of age, but they haven't developed the "magic in the bottle." Then again, Geyserville is a bit of a different animal because of the blend. In my experience, the Paganis get flabby over time.
Despite my few great experiences with older Zins, aging them is not a high-percentage proposition, and doesn't make sense if you like Zin for the blast of up-front brambly fruit.