User blog comment:DragonsHaveMoustaches/Why did Gaius take Morgause to the High Priestesses?/@comment-2190640-20120619184235/@comment-5102537-20120620103758

We don't know if Uther wanted her dead. He only said that he was led to believe the child had died. Gaius said that he smuggled her out of Camelot shorty after her birth, so it actually can't have anything to do with the Great Purge. You're right, it has never been specified how old she is but she is definitely older than Arthur, it's obvious. She can't be younger than him, and in order to be a victim of the Great Purge as a baby, she would have to be younger than Arthur. So I guess we can assume that there must have been other reasons why Gaius took her to the Old Religion. It's a pity that the entire Morgause/Morgana/Vivienne/Igraine-mystery hasn't been clarified.