I originally thought Mufasa was at least a year older than Scar until I read this:
[quote="YFWE"]I don't think there's, say, a years-apart difference between the two. In my opinion, their parents would've just stopped at one litter, especially if they already saw they had a son. There wouldn't really be a huge necessity to keep going, so to speak.[/quote]
I agree with YFWE. It wouldn't make sense for Mufasa and Scar's parents to mate again about a year later if they already have a son to be crowned king one day. Also, the fact is that lions usually mate, and produce a litter (up to 6, I believe) at the same time. Since I ruled out the year age difference, it's safe to rule out a month age difference knowing the parents already have a heir to the throne so there's no need to mate again months later. Finally, I feel it is illogical for their parents to give birth to another cub only weeks or a few days after giving birth to their firstborn, knowing still, lions usually give birth to a litter at the same time, and lionesses are usually pregnant for a few months.
So, the matter of time ranging in the age differences has to range from seconds to minutes, but there's very little in the movies (that I can recall) that could indicate Mufasa is older than Scar by mere minutes or seconds, although you guys have dicussed Scar's jealously for Mufasa for being minutes or seconds older than him, which is interesting.
So, I believe without a doubt, that Mufasa and Scar were born in the same litter, but Mufasa was born first. Whether seconds or minutes older is unknown.