My family was just talking about this this past weekend. My sister turned 21, but at home with the family and close family friends my parents would always give my sister a drink or two, as long as she wasn't driving herself anywhere after. After reading through the Washington laws regarding minors drinking, we found that it technically isn't illegal.
"(3) Subsections (1) and (2)(a) of this section do not apply to liquor given or permitted to be given to a person under the age of twenty-one years by a parent or guardian and consumed in the presence of the parent or guardian. This subsection shall not authorize consumption or possession of liquor by a person under the age of twenty-one years on any premises licensed under chapter
66.24 RCW."
http://apps.leg.wa.gov/....aspx?cite=66.44.270 However, we were trying to figure out what the case is for my wife and I. Technically we wouldn't fall under this exemption, because obviously, we don't live with our parents. Then we were trying to argue that we could be each others guardians and give it to each other, but we don't fall under the legal definition of a guardian.
Then my dad told us that without a warrant no one can come in to see what were doing at home :D
In all reality, it would probably be about a drink per week, if that, so we aren't hugely concerned. But we think it's funny we can be fully independent adults but we can't get a rental car, some hotels, alcohol, or a myriad of other things.