Portions of Washington Initiative 1639 dealing with age requirements for purchase of "semi-automatic assault weapons" take effect January 1. This is the same requirement as existing law for pistol purchases in Washington. New requirements dealing with background checks and safe storage requirements take effect July 1.
The definition of "semi-automatic assault weapons" is as follows:
(25) "Semiautomatic assault rifle" means any rifle which
utilizes a portion of the energy of a firing cartridge to extract
the fired cartridge case and chamber the next round, and which
requires a separate pull of the trigger to fire each cartridge.
"Semiautomatic assault rifle" does not include antique firearms,
any firearm that has been made permanently inoperable, or any
firearm that is manually operated by bolt, pump, lever, or slide
Here is the text of the initiative as passed by 60% in November, 2018: