Claire though about all her problems. She wasn't able to find a girlfriend or boyfriend. She wished things had been like they were before puberty. She wouldn't want to revert to a child, she liked being an adult in some ways, she just wished she didn't have all those hormones rushing around her body.
She though about how she could change things. She checked to make sure the book wouldn't be dangerous; it said it wouldn't accidentally erase someone, and 99% of humans wouldn't remember a thing (the person who made the change will always be one of the few who remember). Changes also only came into effect once the book was closed.
Claire changed a passage in the book about human technology so that humans could reproduce artificially, then she changed human culture so that everyone was castrated/neutered at birth, weather they be male or female. Without ovaries or testicles puberty would be impossible, people would still grow up, she'd still be the same hight (probably a bit taller), and have an adult mind and everything, she'd just never have developed sexually.
She was exited to try being a eunuch. Though very nervous. Even if she didn't like it, she could always go back. A change doesn't have to be forever.