I think a custom expiration policy could solve your issue, as described here or here. In your custom solution you can send a mail / assign a task to the last editor / creator of the document when it is older than the desired lifetime.
You can change a status flag on the document itself that you can use on the UI to filter the documents and / or assign conditional formatting rules / icons (like exclamation mark) to it.
You can create a custom document content type and assign the expiration policy to it (recommended), or you can assign it directly to the the doc. library if you wish to keep it simple.