How old was Vashti when she was summoned to Achashveirosh?
Yalkut Shimoni, Nach 1049 says that Vashti was 12-12.5 when Belshazzar died:
וכשהמלכו דריוש אותה הלילה בלשאצר שהיו אלו הורגים ואלו בוזזים ושתי היתה נערה
And when they made Daryavesh king, that night Belshatzar - some of them were killing [him] and some of them were looting him. Vashti was a Na’arah.
Megillah 11b says that Achashveirosh ascended the throne five years later:
חדא דבלשצר וחמש דדריוש וכורש ותרתי דידיה
One year for Belshatzar, five years for Daryavesh and Koresh, and two years of [Achashveirosh’s, which were followed by the party].
Combining these two pieces, it follows that Vashti was roughly 19-19.5 when the feast started, and roughly 19.5-20 when she was killed.