It’s an interesting question. There’s been previous discussion about improving wiki topics, including adding a post to the topic when an edit is made:
https://meta.discourse.org/t/what-is-a-wiki-post/30801/9?u=jomaxro
I’m not sure what the best solution is. Editing a non-wiki topic does not change the post date, so changing the post date of a wiki topic could be confusing. I do also think that knowing when the topic was created is useful, not just when the last update was.
The edit log icon displays the last edit date, but only on hover. Perhaps for wiki topics only displaying the last edit date would be useful. Something like this could work 
but I fear there will be confusion over the two different dates. Some text could be added, but now the top right corner is getting pretty full. 
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Typing all this out made me think a bit more about this however. An “old” guide is not necessarily a bad thing - it just means the feature/settings the guide is about hasn’t changed. Looking at the subfolder guide, for example, there haven’t been substantive changes since Nov 2016. That doesn’t make the guide any better or worse, it just means that Docker, NGINX, and Discourse haven’t made changes to this area in a year.
I guess I don’t necessarily see the need to make changes here at this time. I still like Sam’s independent idea to include a note in the topic stream when edits are made, but I don’t think we need to adjust how the last edit date is displayed at this time.