Image from The Whisky Obsession
By law, Scotch whisky including blended whisky must mature in a cask for at least 3 years and one day. Single malt is mostly matured 10 or more years. Only oak casks is used since oak wood is breathable and durable.
The origin of the cask is crucial for the taste of the whisky. Some big distilleries like Glenfiddich mainly use American oak that previously used to mature bourbon whiskey. Whereas Macallan and Bowmore are using European oak like Spanish sherry oak.
The place where the maturation occurs have a little bit of influence to the taste as well. Old warehouses are dark and have earth floors and stored in oak beams and stacked on top of each other in 3-6 rows. While modern warehouses have concrete floors so the casks can be moved with forklifts. Hundreds of years ago, the casks were stored lying in the steel racks. Nowadays, the casks are mostly stored upright on pallets. Whisky matures differently depending on the location since there are cold and hot summers, snowy winters, and others have mild climate.
Usually the whisky is filled into the cask with an alcohol content of 63.5%. The alcohol content decreases 0.2-0.6% annually. It is called the "Angel's share". The fluid level decreases 2% every year. Due to the evaporation and the absorbing of the flavours from the cask, the whisky becomes mellower each year. Samples are taken regularly from each casks to find out when the whisky has reached it's prime. The size of the cask is equally important. Larger cask have a smaller surface proportion to the content and fewer flavours can be extracted from the wood. Therefore, whisky matured in large casks must be stored longer in order to reach the same level of maturation. The opposite happens for a smaller casks.
In order to achieve a consistent taste, distilleries must blend their single malts accordingly. Most single malt whiskies are blended from several casks. The age statement on the bottle refers to the youngest whisky used for this particular bottle.