I was just in Canada last week with my 13-year-old.
I had the letter from my wife, but the border agent never asked for it.
I think the older the kid is, the less likely it is that they are being taken against their will, so IMO it seems less of a concern than with younger children.
I HAVE been asked for the letter when the kids were younger.
When I first started taking the kids to Canada, as is common with Canadian government dealings, I could not find or get a straight answer on whether or not a consent letter is actually REQUIRED for entering Canada with minors.
Lots of sites use mushy language saying it is “advised” or “helpful”, but I was not able to find an actual definitive answer on a Canadian government site.