Filter by Modified Date
One way to identify 'old' content is by filtering by "Modified Date". This filter allows you to review content that was modified within a given time period, before or after a certain date and more.
You can further narrow your search by combining this filter with a tag so that you can look at related content that was created within a given time period.
Another way to clean up your Knowledge Base is to look at the related content for a given record. This is basically a search of content in the Knowledge Base to show you what might be related or redundant. If you have access, you can delete or flag content that you think doesn't need to be there.
Deleting content removes it entirely.
Flagging content turns on the flag for review so you can easily find the content in the future.