This comes directly from Dewar's:
"Dewar's Signature whisky doesn't have an age statement because it is made from a selection of very fine whiskies of many different ages. It isn't the age that matters - it is the quality of the individual malt and grain Scotch whiskies that interest us when creating this product. As we strive to ensure we always make it from the best whiskies, the ages of these whiskies will also change each time it is created. Some of the whiskies will be very old, and some will be much younger."
"According to the law, we can only state the age of the youngest whisky in the blend, and that simply wouldn't come close to explaining the richness of the whisky."