According to WolframAlpha, which seems to know absolutely everything, plaid is 497 years old. The site states the first recorded use of the word was in 1512. Not familiar with 1512, here are some notable events: 

James I, king of Scotland was born

The Battle of Ravenna took place

Martin Luther becomes a doctor of theology

The ceiling of the Sistine Chapel, painted by Michelangelo Buonarroti, is exhibited to the public for the first time.

This means somewhere in Scotland there may be a closet with a plaid shirt from 1512 hanging in it and you thought that shirt from college was old.