How long is jack Daniels aged?

At least 4 years. Single Barrel is left to age in the oak barrels the same amount of time as your standard No.7. Single Barrel is kinda explains itself, the bottle you can buy off the shelf is from one individual bottle. That means that more than likely every bottle you would buy would be from a different bottle which would give you a slightly different taste. No.7 is a mix of alot of different barrels after the aging process and the mixture is determined periodically from the Master Distiller of Jack Daniels.