If you were convicted of the DWI when you were 20, then yes, a subsequent DWI would be an aggravated offense. But if you were only convicted of the "Baby DWI," then the new one is not a second for the purposes of elevated penalties.
The information provided in these responses is not, nor is it intended to be, legal advice. You should consult an attorney for individual advice regarding your own situation.
Simply being under 21 at the time of your offense/conviction will not disqualify a prior offense from being considered later. The key is whether the conviction was for an actual DWI for being above the legal limit or impaired, not just for consuming any amount of alcohol.
In Washington if the first DUI was charged as a minor DUI under RCW 46.61.503 then it will not be counted in determining the mandatory minimum sentence that the court must impose. However, it is very likely that the prosecution will consider the minor DUI in connection with making a plea bargain offer. It is also like that the judge will consider the minor DUI in connection with any sentence imposed. You should make sure you have an attorney to help you. If you cannot afford to hire an attorney, you should ask the court to appoint a public defender to help you.