When I was a kid, Scar always looked like the older one to me. The 'big brother' comment always confused me, due to how much older Scar looked. I always associated physical weakness more with older age than youth. He still looks older to me. Though nowadays, I understand that looking older doesn't always equate to being older. Perhaps, as a cub, Scar didn't have his signature gaunt and lanky appearence. He could have looked just like any other healthy cub. Watching his brother undergo training and recieve all the attention from their father would have not only made him jealous, but perhaps stressful as well.
Now, stress can enhance the effects of ageing. I've seen 20 year olds (humans) get grey hair because they were over-worked or having a troubled life. If, in lions, having a darker mane is usually a sign of older age, perhaps Scar developed that through the excelled aging effects of being in a stressful situation. Therefor, it makes sense to me that they would have been littermates and their appearence differs so greatly due to stress.
For those of you (seems to be everyone besides me xP) who are under the the belief that Mufasa looks older, this can be applied the other way around. Perhaps the stress of having the responsibility of running a whole kingdom made Mufasa appear older. His face would develop more strongly due to being in that position and because he had more work to carry out, he developed more muscles - not through age, just physical activity.
I probably got quiet a few facts a little off, but whatever.
Should be an interesting read nonetheless.