The Mishne talked about a giyores who was a naara meurasa who was mezana.
Doesn't a giyores or ger who were converted by Bais Din in their childhood have a choice when they reach bar or bat mitzva age to opt out and not remain Jewish. Couldn't she say if I'm chayeves misa I don't want to be Jewish. Does she have to declare on the day of her bat mitzvah? How long does she have?
The Gemara on 11a says that once she was a Gedolah for one hour she can no longer protest.
The Ran writes that although the Macha'ah does not help when she is a Ketanah, and she cannot know the exact minute of Gadlus to protest then, nevertheless we say that the protest while she is still a Ketanah helps if she carries through with it until Gadlus.
Dov Zupnik
(See also Insights to 11a for other explanations of how she may be Mocheh -MK)