Albert Barnes Commentary Esther 2:5

"There was a certain Jew in Shushan the palace, whose name was Mordecai, the son of Jair, the son of Shimei, the son of Kish, a Benjamite," — Esther 2:5 (ASV)

Mordecai, the eunuch (Esther 2:7, Esther 2:11), has been conjectured to be the same as Matacas, who, according to Ctesias, was the most powerful of the eunuchs during the latter portion of the reign of Xerxes. Mordecai’s line of descent is traced from a certain Kish, carried off by Nebuchadnezzar in 598 B.C.—the year of Jeconiah’s captivity—who was his great-grandfather.

The age of Mordecai at the accession of Xerxes may probably have been about 30 or 40; that of Esther, his first cousin, about 20.