...Master Chief is fifteen/sixteen in Halo 4: Forward Unto Dawn.
That's really strange, when you think about it.
-
John-117 and the other Spartans-IIs had gotten their augmentations and Mark IV armor less than a year prior to the events of Forward Unto Dawn. Dr. Halsey had artificially sped up their maturation before hand. They were fourteen-year-olds that looked like they were in their twenties.