This is a legal requirement in Scotland, the United States, Ireland and Japan among others, and it is the single most useful thing to understand about a whisky label. A 12-year-old may contain a substantial proportion of much older whisky; what it may not contain is anything younger than twelve.
When the clock stops
Maturation counts only while the spirit is in wood. Time in glass does nothing at all — a 12-year-old bottled in 1990 is still a 12-year-old today, however long it has sat on a shelf. An old bottle is not old whisky, and the two are constantly conflated at auction.
No age statement
An NAS bottling carries no number. It is not necessarily young, but it removes the constraint the youngest cask would otherwise place on the blender, who can then use a small amount of very young spirit alongside much older stock. Whether that is a gain in flexibility or a loss of information is exactly the argument the category has been having.
Older is not automatically better
Extended time in an active cask can leave a spirit dominated by wood, with the distillery character buried under tannin and oak extract. Age is a fact about a whisky, not a verdict on it.