At my regular tea webshop, you can always see what year and month the tea was harvested. Just recently they have brought in this spring's harvest of Darjeeling. But they also have tea from 2011 (in this case a black tea). How does harvest time matter with teas? When is a tea considered "old"? Do they lose flavour when storing for more than two years? Would a more recently harvested tea be considered a better buy or does it depend on taste and type of tea?
Harvest time can have a large effect on the quality of the tea. First off, whether it was the first harvest of the year or a later one will greatly change the character of the tea.
The dates can also be used similar to the vintage dates of wine. If you got a tea from a specific region and know the date of the harvest, if you find another tea from the same region and same date, it will have been grown under similar conditions.
Also, since black and oolong teas have been oxidized, the oxidation process will continue to some extent as the tea ages, further changing the flavor. The date can be used to predict freshness, and also to find a tea that suits your tastes better (for instance, I know that I like many black teas better after they have aged for a year or two).