I have no idea what I'm looking at here, lol.

Check this out tho: I don't think Chief should have looked as old as his eyes looked at the end of the game anyway, because we already are looking younger and younger, and in the future there will be even more things that slow the aging process. Halsey would want her Spartans healthy for as long as possible, and I can easily imagine them being genetically altered to not have their cells replicate improperly with bad code as fast (which is, after all what aging is, right?)