Stripped lowers are marked on the 4473, and sold as, Other Firearms. Not pistols.
You can't purchase a stripped lower as a "pistol" on a 4473 because it's not one. Go
here and look at question 16.
It's a misconception that 18-21 year olds can't buy receivers because they might be, or are pistols. The actual law is that people under 21 can't buy a Firearm from an FFL. There is an exception to that for Rifles and Shotguns. If a firearm isn't expressly a Rifle or Shotgun, you must be 21 to buy it. (from an FFL, under federal law) 18 USC 922 (b)(1)
That's purchasing from an FFL, private sales vary by state.
Could an 18yo, go to say, Idaho and buy a lower there??
No, that's illegal under federal law. You can't buy a firearm in a state that you don't reside in, and he/she can't ship it to an FFL in WA and transfer it there because it's not a rifle/shotgun.
An 18 year old CAN (most places) buy an 80% receiver, mill it out and manufacture a firearm for personal use. It seems that would fall outside WA's new law.