Most Saké is intended to be drank within a year or so of purchase. But there is a rare percentage of Saké that is intentionally aged, and it is called Koshu. This is not something new but actually has a long tradition, though it waned for some time until recently. It is now becoming more popular but still remains a rarity.
For more information on Koshu, check out the SF Gate which has an informative article, Exploring sake's dark side on the topic, including reviews of five different Koshu.