LCR follows the Handbook and the developers are given the "business rules" by the same. I don't see any exceptions for age rules for those classified as "not accountable". Note that according to Handbook 11.4, she's probably going to be forced into Relief Society when she turns 19.
I'd first check and make sure she has been classified as "not accountable" on her membership record and not just "not baptized". Assuming this has been done, I'd suggest contacting GSC to see if they have any suggestions. GSC does have access to more controls than is available to the ward clerk. You may want to email to make sure they have time to research the answer, instead of trying to get you something during the phone call.
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