Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hi Mrsc328,
Based on your description, if you want to delete the deleted items from Deleted Items folder when this items older than 18 months, you can create retention tag and assign retention policy to the Deleted Items folder. Note: it will take effect in 7 days when you assign policy.
See detailed step for your need: https://technet.microsoft.com/en-us/library/dd297955(v=exchg.160).aspx
Attention: if you want to setup retention tag or retention policy, you should have Office 365 account (admin permission)
Thanks,
Richard