if you're questioning her age just by her looks, she is a (spoiler)
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in her world there are semi-immortals, like the guild master that was just introduced. Sherly is also a semi-immortal, and those types have different power sets. In Sherley and the guild master's case, their appearances do not age even if they do. The guild master is a few thousand years old btw
anyone who have read the novel? where is the father of these children? is she even their biological mother?