It passed... WA is new California now 
Now it is a matter of finding out exactly what this means.
The age limit is self explanatory. What I'm wondering about is the safe storage, testing requirement, enhanced background checks, etc. provisions in the bill. There has been A LOT of misinformation provided by both sides and I am eager to learn what is what without the included chaff.
You can search the text of I-1639 online. There's a lot of text and it takes time to read and compare with current laws. My biggest concerns are:
(1) Every semiautomatic rifle (including Marlin .22lr) is considered "assault rifle" now. With Dems strengthened majority in WA State Senate and House they can pass now any law to ban or restrict "assault rifles" (I'm afraid they will ban "high capacity magazines" first.)
(2) Background check will include now local police/medical history check (you'll waive privacy on your medical records when you buy "semiautomatic rifle"). They told that it's similar to the background check when you buy handguns, but with CPL (concealed pistol license) you avoid local check for handguns and only need to renew your CPL every 5 years. Having CPL won't help you if you buy rifles - they will make local background check with every purchase. And they will charge you with additional $25 fee for this check. They will also create some kind of "assault rifles registry", so that local authorities can check who owns guns on yearly basis.
(3) Requirements to complete "Rifle safety class" every 5 years. It's not clear how they will force it for people who already own rifles.
(4) Safe storage requirements. While it is not explicitly defined what is considered "safe", they can make changes in future "what is licensed or considered secure". So, if you already spent your money on gun cabinet or safe (being responsible person) you might throw it away cause it is "not secure enough" in future
In general, I feel bad that this initiative was approved with 60% of voters. It would be extremely hard to overturn it in courts if anyone ever try to do.