Yeah, but as was said before, he's been referred to as ''old'' or ''Old King Graham'' since KQ4. So, I think in the King's Quest universe, what is referred to as old would be relatively young or middle age in the modern world. Remember, King's Quest DOES take place in a medieval world, albeit a fantasy one. A man in his 40s or 50s would definately have been considered old in those times, as most people didn't live to that age, and I guess Graham going gray/white early would also contribute to him being referred to as old.
I mean, in the King's Quest universe, is a character's hair color really a big inconsistancy? Like in KQ1 looks like he's suffering from Jaundice lol he's like a little yellow man, but by V he's of a reddish hue, and by MOE and TSL he looks like a normal man.
And yes, I think MOE's Graham is inconsistant with himself. In the opening he looks pretty fat and doesn't look all that old except for the white hair (which some people go prematurely white, I have an uncle who is in his 40s and has snow white hair) and if you look closely Graham has a mustache. And yet in the ending cutscene he looks frail and skinny, almost anorexic, and aged.