She must have been at least 21, as Ahashverosh incited her to the drinking party, as it says that drinking was “according to the law”
Unless you want to say that she was under 21 and that is why Vashti refused to come.
But you would not think that the king, who did everything legally, would violate the law?! The tirutz is that she was one day before 21 and they had a healthy machloket whether to count previous night as the day, Vashti following a goyishe minhag of starting day from midnight hatzos. So she lost her head exactly at hatsos, otherwise after that she would have been able to come to the party.
So, we know her age exactly – one hair before 21.