Check her driver's license. In Washington, you can be convicted of child molestation 3 or child rape 3 if she's under 16 and you're 48 or more months older than she is. Even if she lies about her age and it's completely consensual, you can end up with a conviction that will greatly impact the rest of your life.
Sixteen is the "age of consent" in Washington. It is not a crime for anyone to have consensual sex with a 16 year old, with three important exception:
(1) If the minor is 16-18 years old and the person is at least sixty months older, in a significant relationship to the victim, and abuses a supervisory position within that relationship in order to engage in sex;
(2) If the perpetrator is a school employee who has sexual intercourse with an enrolled student of the school who is 16-21 years old, if the employee is at least sixty months older than the student; or
(3) the person is a foster parent who has sex with his or her foster child who is over 16 years old.
If the minor is 14 or 15 years old, it is a crime for a person 48 months older to engage in sex with the minor (and yes, they can and will prosecute you if you're 48 months + 1 day older)
It is not a defense that the sex was consensual (obviously, if sex is not consensual it is rape regardless of the ages) or that you were mistaken about age (people do lie about their age and it's your responsibility to verify the person is of legal age before having sex).