User blog comment:Emily Windsnap/How old are the Merlin characters?/@comment-72.192.17.10-20140810123139/@comment-5674726-20140810123937

Morgana's brief reigns can't be taken as evidence of birth order, one way or the other, since she took the throne by right of conquest, not because she was Uther's firstborn. I side with her being younger than Arthur because it seems much more plausible for a grieving Uther and lonely Vivienne to have had a one-night stand after Ygraine's death than that Uther cheated on his beloved wife.

I don't think that there's any question but that Morgause is older than Morgana.