My reason for her being so young was because she was brought to the tower pretty young and 20 years is a long time, if you behave and are on top of things it should be possible, since the only requirements are learning your weaves and braving the trials through Novice to Aes Seda. Age wasn't really a factor. Her power also isn't nearly as strong as Nyneave's. She is weaker than Egwene and Elayne. Her power level istelf wasn't the issue.