Rather than notifications to users for old pages, a customisable page banner auto inserted where it meets some criteria (Last edited +90 days ago) would be useful as it'd not bother authors, but lets the reader know how valid the page is right away.
* <1 hour - "This page is being actively edited"
* >90 days - "This page hasn't been edited in a while and may need to be updated"
* >365 days - "This page is over a year old, it may need to be archived or rewritten"
* > 1000 days - "This page is a fossil and should only be read by archaeologists"
...and related to that:
Provide mechanism for space/global administrators to enable banners on ALL space pages indicating if pages are unused or expiring - ie "This page hasn't been updated in ages, it'd be great for you to update with new information if you have it!"