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DearCustomer- If the charge was dismissed it was not a conviction so I can't see how it would be part of your record. In any event you were honest and told them about the improper operation so if somehow the other charge comes up you can honestly say that it was the improper operation and not the other.
In any case, I doubt anyone is going to worry about a charge like this if it was dismissed. Generally the two areas employers really care about are crimes of dishonesty like theft and crimes of violence and especially sex offenses. Drinking under age, although it is technically a crime, is something many people receive and does not affect their overall character or trustworthiness.