Sherry is aged by an original system called "criaderas y solera" in American oak casks of 600 liters, filled to 5/6ths capacity. While in other Denominations (D.O.) the casks are hermetically sealed, in Jerez they are open to allow the wine to be aired by the southwest breezes which, when in contact with the natural yeasts of the Palomino grape, form a veil of growing yeast or "flor" that isolates the wine from the air, thus giving it its characteristic nutrients, aroma and taste.
Sherry butts (casks) are stacked in at least three rows. The first row (solera), that is nearest to the floor, contains the oldest wine ready to be drawn for bottling. The quantity that has been taken from the bottom row (solera) is replaced from the row above (1st criadera), which is refilled in turn from the row above (2nd criadera), and so on until the youngest criadera is topped off with carefully selected "new" wine.
All sherry wines must age for at least three years - the minimum for Finos and Manzanillas. Amontillados are left to age for (at least 5 years), and Olorosos 7 years.