Similar to Colorado, if you're a medical patient with an authorization, you can acquire medical marijuana if you're 18, 19, or 20 years of age, and with a provider if you're younger.
It's a felony in Washington, which is a lot more egregious than the other states I just heard from, for a minor to either possess or try to purchase cannabis. I'm actually surprised that hasn't been changed over the last few years, but it's pretty rough in this state in respect to that.