Well Friday is the best night for
"The Underground" which is below Neighbors Nightclub but it typically seems to struggle to draw a crowd. While Fridays have been the best it is also an early night so get there early. Neighbors Nightclub is open to the under 21 crowd after 2 am till 4 am so usually a line will form.
The Seattle Party Boys has hot private events but it is typically by invitation only or you have to know someone or you have to be very pretty.
Penalties for allowing minors into bars is huge (even if they have a fake ID) so everyone gets carded at most bars except during off-peak hours. And for those from California temporary paper ID's are not accepted without having both the cancelled picture ID and the new paper registration since expired ID's are not accepted just like the TSA.
As an added note - Seattle now has a smoking ban which is the greatest thing to ever happen to the scene since you can go home without smelling like an ashtray plus a lot of boys spend a lot of time hanging outside of the club to smoke which is nice for those under 21.
Best advice is to wait a few months until you are 21.