How do you get a hotel if you're under 19?

I came across the same problem. None of us live at home any more, but all of the hotels operated by Whistler require you to be over 21… which we aren't

There were only three rooms available for 19 year olds, starting at over $200 a night. I dunno about you, but that's not us.

The solution was the local hostels. Turns out, they're way cheaper anyways ($25/person/night), and they didn't ask about age at all. just google Whistler hostels, and you'll find everything you need to know.
Another bonus: most have free parking.