I'm assuming by "dating" you mean having as sexual relationship. The age of consent in Washington is 16. Unless there is a teacher/student, coach/player, etc. relationship or other mental capasity factors not disclosed, this would not constitute statutory rape.
If the teenagers have sex before the younger one turns 18, the older one over 18 has committed statutory rape, for in Florida, the age of consent is 18.
Even if the person under 18 consents, that person is not deemed to be old enough to make a sound decision in this area of life.
If the 19-year-old boy has sex with his 17-year-old girlfriend and law enforcement finds out, will the boy be prosecuted? Likely not, for the younger the girl (could be a boy, but typically is not) the greater the odds of prosecution. Plus, the closer the ages of the two, the less likely there will be prosecution.
I am not condoning a sexual relationship, and I am not condoning that the two get married, but if the two were to get married, sex before the age of 18 would not be prosecuted.