Final answer:
The legend of King Arthur was first written down in the 9th century, making it over a thousand years old. Sir Thomas Malory's 'Le Morte d'Arthur' is one of the important works associated with the legend.
Step-by-step explanation:
The legend of King Arthur was first written down in the 9th century, specifically in the 800s. This makes it more than a thousand years old at this point. One of the important works that contributed to the popularization of the legend is Sir Thomas Malory's 'Le Morte d'Arthur', written in the late 15th century.