You can find your answer in the Handbook or the
Wiki:
"A person who is nine years or older who has a membership record but has not been baptized and confirmed is not considered a member of record. However, the bishop keeps the membership record until the person is 18. At that time if the person chooses not to be baptized despite being given every opportunity, the bishop, with written permission from the stake president, may cancel the membership record. He should not, however, cancel membership records of persons not considered accountable because of mental disabilities."
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