User blog comment:PlantyThePottedPlant/Random Topic Blogging III: Blog Harder/@comment-1659929-20130513233826

NON PnF fan theory:

Mufasa and Sarabi are middle age when they have Simba.

My version:

Mufasa and Sarabi were unable to have children, or more likely, the cubs didn't survive the birth, or died because they were too weak. So, if anything was to happen to Mufasa, Scar was promised to be king. But when Simba was born, that changed. Hence the line, "I was first in line, until the little hairball was born."