When a whisky is bottled, distillers often mix whiskies of different ages together to produce their desired flavour profile. This helps balance any flavour differences between years and barrels to produce a more consistent product.
The age of the youngest spirit contained within the bottle is then printed on the label— ie. a whisky bottled from a mix of 10 / 12 / 15 year old whiskies would be labelled as a 10 year old — we call this an Age Statement.
A bottle containing 3 / 8 / 12 year old whisky would either be labelled as a 3 year old whisky, or no age statement will be printed on the bottle. Traditionally a younger age statement is seen as inferior, this is no longer the case.