I grew up an hour from the border in Canada. We used to cross all the time
without letters up until I was in HS in the 80's. It was probably mid to late 80's that the border officers started to ask if we had permission to cross without parents or if only one parent was present. After 9/11 things got really strict in that they wanted notarized permission letters. I have crossed a few times with a letter and usually get asked to see it as a formality. In summer of 2007 I was almost grilled by US Customs heading back to Utah about where the mother was and why I was traveling with my kids by myself. My wife had similar treatment by Canada Customs going to visit her parents with the kids.