2 answers
Hi Joseph,
By Case Age, may I know if you are referring to the Age field on a standard Case report? If so, then it won't be replaced by enabling the Business Hours Age field. It would have to be a different field added to the report.
https://help.salesforce.com/articleView?id=Business-Hours-Age-field-behavior&language=en_US&type=1
I hope the information above helps!