I can't agree that this helps show what he looks like as an adult. Being trained in nothing , but war since 6 years of age ,then battling human rebels immediately after, then the three decades of nonstop war against the covenant. Witnessing so much death, his allies, friends, family(SIIs). And being responsible for arguably just as much death. All that immense stress changes a person even their appearance.
Ive witnessed stress physically change people. But I could only imagine what all that would do to a person. Even people as stone cold as the SIIs