How old is masterchief?

Oficially he's about 42 (earth) years old. However during slipspace (faster than light) travel, he, along with most other people aboard a ship, are put into cryogenic stasis. This stasis almost completely stops all cell generation, and degeneration, so a person's body does not physically age while in this state. Considering slipspace travel can take anywhere from a couple days, to about 6 months, it is difficult to say how old his body really is. You can assume though, that it is at least in the early to mid thirties.

By what Halopedia says, he's 21 year old in the standard 21st century time.

http://halo.wikia.com/wiki/Master_Chief#Age

If the year he died is 2553, and was abducted at the age of 6 in 2517, then he is 36 years old.