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In this age group, what could be causing this confusion?

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The commonest cause would be delirium secondary to infection. Drug interactions or toxicity should be considered especially in those prone to polypharmacy. In someone with no known history or infective signs, CT head can be considered to look for structural causes or in rapid onset cases, a stroke.

Meningitis could be another cause of altered mental state - how would imaging help diagnosis this?

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There is limited role for imaging in diagnosing meningitis. The mainstay of diagnosis comes from CSF analysis, and some use CT prior to lumbar puncture to exclude mass lesions. The growing impression however is that unless there is "hard neurology" that makes you think there is a reason for raised ICP, lumbar puncture should be safe and takes priority to speed up commencing treatment.