Exporting a paged report with Age or Age Description field will only export the first page not the entire report content as expected. Both 'Export To CSV' and 'Export To Excel' will have the same result.
Workflow 8.5 RU1
ServiceDesk 8.5. RU1
This issue is fixed in Workflow and ServiceDesk 8.5 RU2.
Workaround until then is to avoid using Age or Age Description fields in paged reports that are being exported.