is it true that the oracle of Delphi, instead of being a virgin maiden was a post menopause women?

Yes, it was an older woman.

"The priestess of the oracle at Delphi was known as the Pythia. Apollo spoke through his oracle, who had to be an older woman of blameless life chosen from among the peasants of the area."

The oracles of Delphi were usually chosen as hermaphrodites (both male and female) and the head priestesses were past the child baring years pre and post menapausal women. The vestal virgins were the "purists" of Delphi. Untouched by man or god and pure of heart. These were the hand maidens, dancers, praise singers, and focus entities for the Oracle to think freely on things. Only very few of the known Oracles of Delphi were post menapausal priestesses but were lifetime learners of all the beliefs and were usually empaths or seers.

I think you may be confusing the Delphi priestess with the vestal virgins. The vestal virgins needed to be virgins & were released from their duties at the age of 30. The oracle was a highly respected priestess chosen from the community.