I saw this article a few days back and didn't see it here so I thought I'd share. It mentions the earliest known example of British/ Scottish tartan found on a single piece of fabric from near the Antonine Wall. Of course there have been earlier examples of tartan clothing from continental europe and even the body of the 3000 yr old Cherchen man in the Tarim Basin of China.

http://www.falkirkherald.co.uk/news/falkirk-tartan-is-the-oldest-in-britain-dating-back-to-roman-times-1-3994264