Originally Posted by
6rugrats
Those letters are very useful to have when the airline requests them or they will deny you boarding.
Being asked for such a letter by the airline and parental non-possession of such letter being grounds for the airline to deny boarding to the EU is so very rare that I am comfortable with the idea of it happening on the order of less than 0.5% of the time (ie less than 1/200 times and not even that often) even when the travel is via Canada and involves CBSA passport control in Canada.
17 year olds in the EU routinely travel internationally into and out of the Schengen zone without their parents and do so without any parental permission letters. The EU passport control is more likely to care about sufficient funds, onward travel and health insurance coverage but that is so rarely asked of US citizens that I would be surprised if that happens.