Why is Red Hulk's hair that of a young person's when Ross's hair is that of an old person's?
The hair changes from grey to black during the transformation. We see this during Hulk Vol. 2 Issue 23, "Who is the Red Hulk?", in the World War Hulks storyline. It is revealed that Red Hulk is indeed Ross and we witness the transformation a few times including a full body shot:
We also see his transformation in the below image but I'm not entirely sure where it's from.
As for why I think it's because Red Hulk, among other Hulk's, are incredibly strong and have a regenerative factor. It shows how strong Red Hulk is in addition to the obscene muscles.
It's also worth noting that everything about Ross changes when he becomes Red Hulk: size, muscles, eye colour, hair colour etc. He also loses hair in several places, most notably his facial hair but I don't spot hair anywhere else on his body to be honest. It just signifies the difference between the two sides of the same character.