(SPOILER)At the VERY least, as the question says, Mika is fifty.

Lulu: Maester Mika is the leader of all the people of Spira. He's come all the way from Bevelle. The tournament is being held to honour his fifty years as maester.

This of course assumes he was born a maester and worked his way up to Grand Maester later, since Lulu doesn't say it's been 50 years since he became Grand Maester.

However, his actual age is 99, as stated in FFX's Ultimania guide.

I choose option E.