Age.

Age. The length of time during which, after distillation and before bottling, the distilled spirits have been stored in oak barrels. Age for bourbon whisky, rye whisky, wheat whisky, malt whisky, or rye malt whisky, and straight whiskies other than straight corn whisky and straight American single malt whisky, means the period the whisky has been stored in charred new oak barrels.

Source

27 CFR § 5.1

Scoping language

When used in this part and on forms prescribed under this part, the following terms have the meaning assigned to them in this section, unless the terms appear in a context that requires a different meaning. Any other term defined in the Federal Alcohol Administration Act (FAA Act) and used in this part has the same meaning assigned to it by the FAA Act.