Sherry Vinagre Shelf Life
Other than my homemade herbed vinegars, l have found the basic products, wine, sherry, etc. do not seem to change much over time. l have so many that years may go by before a bottle is finished. While their aromatics may decrease a bit with time, the flavor still seems spot on. If they are very delicate, l fridge them from opening on, but that is the exception.
krsmav's explanation is the most obvious one, but as with balsamic vinegar, unless you've got a (very expensive) bottle of traditional, legally regulated "Vinagre de Jerez DO", "Solera 30" on the bottle may not really mean anything at all.
Both sherry-the-beverage and sherry-the-vinegar are produced by the solera system. So without knowing more about everything on the label, "Solera 30" could mean it's young vinegar made quickly from a 30 year old solera of sherry, or it could mean the vinegar solera is 30 years old. It could be "Solera Number 30" of undetermined age, or it could be a "brand name" on a bottle of "industrial vinegar" made by a quicker, modern process in which case it really means nothing at all - just questionable marketing with the hope that purchasers will think they're getting something they're not.
Agree with krsmav that the shelf life is infinite. I have a bottle of Sherry Vinegar that's been in my pantry for longer than I'd care to say. I'm sure the Food police would say it does have a shelf life and should be tossed. I used it about 3 days ago, and I'm here to tell the tale.
Shelf life: infinite.
A solera is a series of barrels. When the oldest is tapped, it's refiled from the next one down the line, so the average contents gradually get older. The date is when the oldest barrel was put down. Thus a bottle of solera sherry vinegar will have some 30-year-old vinegar in it, to mellow the flavor, but lots of newer stuff. See <http://en.wikipedia.org/wiki/Solera>.