Fettercairn

[Hirsty]

18-30 is a general rule of thumb. Each distillery differs, for example, I interned at Edradour and they have some wonderful younger products.

The 15 year mark is when the majority of the residual sugars have been extracted from the wood but there are still some there. You are getting decreasing returns but it can add complexity.

Charring also effects the timescale as does moving to a finishing cask.

It is also worth considering the yeast or yeasts used for fermenting, the malt used (as in barley), water chemistry, and even the conditions in the bond.

Just as an aside, I am not a huge fan of single cask - one of the aspects of the artistry of being a distiller is the ability to blend different casks together to create the desired end product.