I've honestly never heard those two being linked before. That would seem to imply that Canada, with a drinking age of 18, would you have a a higher drunk driving rate than the US, and I wonder if that's the case.
In most of Canada the drinking age is 19 (only a few provinces are 18), but it’s also slightly harder to get your drivers license in Canada. You’re required to take a written test and then two driving tests in order to receive a full license.
Sure, OK, but that's still at least two extra years. We didn't used to have the two driving tests, and in Alberta we don't anymore - you transition off GDL after a set time without a test, but all the GDLs were instituted relatively recently, so we should be able to look at data 20-30 years ago to see data matching that claim, right?