Pepper spray is legal in Washington with a few restrictions.
Section 9.91.160; allows the sale and possession of "personal protection spray devices" like "mace, pepper mace, or pepper gas". Also, age restriction (18+), or 14+ with parents consent.
To sum it up... Legal for self defense, must be 18 years old or older, or 14+ with parents permission.
Below are recently purchased pepper sprays by Washington state residents.