There is a theory on halopedia about Johnson being a Spartan I, it even has a theory about his age. Because a UNSC ship goes into slipspace the crew is put into cryogenic stasis (basically freezing them), they do not age. So someone who travels alot in slipspace would actually be physically younger than if they had not been in stasis. So Johnson could be a Spartan I, and only be about 50 years old. Its one of the many mysteries of the Halo universe