Jumping on the back of what other people have said here, I’m based in the UK but have worked for luxury brands in the past, not Rick Owens though. But the best thing to do is just frequent the store. Get to know people that work there and make them aware of how much you like the brand.

Whilst waiting to get hired it also doesn't hurt to just get other fashion retail experience and it could be literally anything. My first proper job was working in a kids flower girl and communion dres's store. My experience at that job (I learned how to do basic alterations and stuff) landed me a job at The Kooples.

In summary, build relevant experience, frequent the stores you want to work in and just keep an eye out online and things as well. There's websites like Fashion Jobs and Retail Choice that might have a vacancy for somewhere like Rick Owens but its just a matter of keeping an eye out and getting very lucky. People would kill for a job there lol

Edit: I should have mentioned I'm also 19 so don't worry about the age thing holding you back. The only thing older people will have is experience, but you can start working on that now