You may be correct. Possession of alcohol by a minor is a misdemeanor under Wahsington law. RCW 9A.04.080(1)(j) states that no misdemeanor may be prosecuted more than one year after its commission. What you want to look for is when the charge was filed by the prosecutor. If it was filed prior to the one year statute of limitaitons, then it will likely stand up. However, if it was filed after the one year period, you likely have a defense to the charge.
In addition to the statutory cut-off for the prosecution to file charges, there may be a common-law motion to dismiss for pre-filing delay. It's not as though this case was so complicated or took a lengthy investigation before it could be filed.
Even if the prosecutor's office met the statute of limitations, your daughter's attorney should look into the second prong of this motion to dismiss.
Yes, it does seem a little late, but the prosecutor has filed the charge within proper timelines. MIP is a gross misdemeanor and gross misdemeanors can be filed if filed within two years of the incident. However, a review of the facts and procedure thus far might reveal other possible avenues of defense.