Answer
According to Jewish rabbis, referencing: Bereishet Rabba , Queen Esther was either 40, 75, or 80. The ages are a combination of three opinions
Answer
Esther 2:2 says that young virgins were brought before the king, and we find that Esther was one of these young virgins. According to widespread customs of the time, Esther would have been in her early teens, or even younger. Xerxes ruled for a total of about twenty years, so in this story Esther could have been Queen into her mid-thirties.
We can not look to the historical record for Esther's age, because there is no historical record of Esther or of her predecessor, Vashti, and Queen Amestris is accepted by historians as Xerxes' only wife for the first several years of his reign.