Is this the sign you saw?
If so, regardless of where it is posted it follows 9.41.300
I have seen many restruants with a coctail lounge place them in the incorrect spot. I ignore them as they are incorrect and I am not breaking the law by being in the restruant portion.Weapons prohibited in certain places — Local laws and ordinances — Exceptions — Penalty.
</B>(1) It is unlawful for any person to enter the following places when he or she knowingly possesses or knowingly has under his or her control a weapon:
(d) That portion of an establishment classified by the state liquor control board as off-limits to persons under twenty-one years of age;
On a side note: I was in Vancouver, WA today at Red Robin and they did not have any signs in the coctail lounge portion of the restruant. I do not see that RCW 9.41.300 states that they must, however, on the WSLCB site they say that they are required.