The most reliable dating I have for the concept of wearing one's mother's tartan reaches all they way back to 19th century Scotland. I've never come across anything earlier in print, although a lot of 20th century academics and FSA scot-types seem to have spilled a sea of ink justifying the practice by reaching back into the mists of Scottish history.

My personal belief is that while a case can probably be made for Pictish practices, in all likelihood the wearing of one's mother's tartan wasn't an issue until the Victorian era when some gentlemen felt obliged to explain why, when their name was Schultz, they were wearing "Grant" or some such other tartan.