User blog comment:Morganamagic7/Siblings Age/@comment-5674726-20130220185702

I agree with you on the birth order but not on the ages.

Arthur was twenty in the first episode, since he comes of age in the ninth.

Given the way Uther's devotion to Ygraine was presented, it seems highly unlikely that he was unfaithful to her so I think that the most probable scenario is that Morgana was conceived after her death, and was the product of an affair between a grieving Uther and a lonely Vivienne. I think that she was older than sixteen, though; the only thing that could potentially support her being so young is that she's unmarried. Her dynamic with Arthur is not that of somebody four or five years younger, they strike me as being very close in age, no more than a year or two apart. I'd estimate that she's eighteen or nineteen in the first episode.

Morgause is the mystery because she is older than Arthur but had to be smuggled away as an infant, before the Great Purge began.