This is primarily a something to watch for in Scotland, although all of the whisky across the world follows essentially the same rules.
Lew Bryson really described this incredibly well in his book “Tasting Whisky”.
Age Statement Whisky
When a whisky is bottled with an age statement (Dalmore 12yr, Laphroig 10yr), that doesn’t mean it is 10 yr old whisky. That means it’s at least 10 yrs old.
When a master distiller is creating the whisky to represent his distillery, he will use different barrels mixed together in order to create the specific and consistent flavor profile of his signature taste. But if it says 10 yr old that means the youngest barrel used during the blending process was 10 years old. It could, and very often does, contain other barrels that are much older.
Example? Here’s a brilliant photo from Lew’s book showing the possible breakdown of a barrel choices in a 15yr old whisky.
Whisky Expressions
When a whisky is an “expression”, that means that the distiller made the choice to pick any barrel that suited him in order to work towards a flavor he was striving to create. Very often it will be within a specific age window in order to keep the flavor profiles in a general direction and make them easier to “marry”.
For example, Jura’s “Superstition” is an expression of the Jura Distillery created using barrels between the ages of 13 to 21 years. That’s quite the gap.
Glenrothes is another distillery that does really lovely “expression” based releases.
The point?
The bottom line is they are both art forms. Both of them are dependent on the discretion and taste of a master at his craft.
Drink what you like.