I have been trying to calculate Mordechai's age, and according to my estimates he must have been around 100, if not older, when the Purim saga begins.
Are there any textual sources which provide the precise age of Mordechai during the Purim story?
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Second Version of Ibn Ezra on Esther 2:6 Calculates that Mordechai was at more than 90 years old when Achashveros married Esther.
Even if Mordechai was one month old when he left Israel, he would have been 92 when Esther married Achashverosh.
According to the calculations of Isaac Samuel Reggio (Italy, 1784 – 1855) in his מפתח אל מגילת אסתר, based on adding up the various reigns of Persian kings throughout those years, Mordechai was a modest "103 or 104" years old at the time.
Although this is not a Midrashic source, this does satisfy the OP's request for a "textual source" as to the age of Mordechai.