He has no given age. If he were human, at the end of Monsters University, he could have been 21 or 22 when he was employed in the Monsters Inc mail room.
There no way of knowing how much time passed between his mail room job & the events in Monsters Inc, but it can be guessed [again, if they were human] they can be anywhere between mid 20s to mid 40s.
Billy Crystal voiced Michael "Mike" Wazowski .
Billy Crystal provides the voice of the green monster named Michael "Mike" Wazowski who also makes brief appearances in Finding Nemo , Cars , WALL-E and Toy Story 3 .
Mike Wazowski.
Mike Wazowski.
Michael Wazowski, commonly known as Mike Wazowski or just Mike. He was voiced by Billy Crystal.
Billy Crystal voiced Michael "Mike" Wazowski .
Mike Wazowski has written: 'M is for monster' -- subject(s): Alphabet, Juvenile fiction, Monsters
Michael Wazowski, commonly known as Mike Wazowski or just Mike. He was voiced by Billy Crystal.
Billy Crystal provides the voice of the green monster named Michael "Mike" Wazowski who also makes brief appearances in Finding Nemo , Cars , WALL-E and Toy Story 3 .
Mike Wazowski.
Mike Wazowski.
Michael Wazowski, commonly known as Mike Wazowski or just Mike. He was voiced by Billy Crystal.
James P. Sullivan and Mike Wazowski.
"Michael "Mike" Wazowski" , voiced by Billy Crystal .
James P. Sullivan and Mike Wazowski.
Henry J. Waternoose, the third generation of Waternoose's that have run Monsters Incorperated.
John Goodman and Billy Crystal, respectively.