User blog comment:Emily Windsnap/How old are the Merlin characters?/@comment-4190137-20120504080114/@comment-4190137-20120505115409

It is an accurate guess. Morgana is presumed to be older than Arthur, and Arthur turned 21 in Series 1 (Coronation is Excalibur), but it really depends on the time skip in Series 5. And also, Nimueh was probably 100 when she died!