On Google Flights I found a roundtrip on Air Canada for 3 passengers. One passenger is 15 years old, which is an adult on Google Flights. When I click through to book on Air Canada, the AC website gives me an error when I enter the birthdate of this passenger because the passenger is not yet an adult. So then I changed the search on Air Canada's website (which classifies 12-15 as "Youth") to 2 adults and 1 youth, but AC's website does not return this routing as an option. The only way to get this routing, it seems, is to go via Google Flights. If I mark this passenger as a "child" on Google Flights, then I still get an error when entering the actual birth date for this person on AC's website because a 15-year old is not a child per AC's classification.

Is there a workaround?