62 <edit> That is his true age but you have to remember that he has spent alot of time in cryo so he is 62 logically but his body is younger. Its complicated.
It should also be noted that Master Chief uses a walking stick. Time acts differently in slip-space, making his true age difficult to calculate from the dates given in the books and games.
He should have a real age of 42 but a body age of approximately 21, according to the halo wikis.