The exact age of the oldest surviving samurai sword is difficult to determine with certainty, as the specific dating of ancient swords can be challenging due to various factors such as preservation, historical records, and changes in sword styles over time. However, there are swords that date back several centuries and are considered among the oldest surviving examples of samurai swords.

One such example is the "Fukuoka Ichimonji Sukezane," a sword designated as a National Treasure of Japan. It is believed to have been crafted by the swordsmith Sukezane in the Heian period (794-1185) or the Kamakura period (1185-1333). This would make it potentially over 800 years old.

Another notable example is the "Sanjo Munechika," attributed to the swordsmith Munechika, who lived during the late Heian period. The exact age of the Sanjo Munechika is difficult to determine, but it is considered one of the oldest extant samurai swords.

It's important to note that the oldest surviving samurai swords are rare and highly treasured artifacts, and the age and origins of these swords can be subject to ongoing research, historical analysis, and further discoveries. New archaeological findings and historical records can contribute to our understanding of the ancient origins of samurai swords.