This guide will show you haw to set your Folder Auto-Clean settings:
After logging in, go to Settings
Under Settings, go to Filtering then Folder Auto-Clean.
Make sure to select the Radio button that says "Override auto-clean settings for this account"
In the Rules tab click the Add Rule button
From here you can select the folder you would like to clean and make sure to only use the Date Type.
This will delete messages that are older than the date set.
Be aware that this will delete messages permanently
This is what you will see after you save the rule: