Currently I have been going through and making sure pages are categorized correctly, and I agree userpages don't really cause an issue with the search to a noticeable extent. My issue with deleting a userpage is what if that user comes back to Fedora, then he would not be happy if we deleted the page he took time to create. If a user wants his page gone he can add {{delete}} to it and it will flag that page in the please delete category. I have come across a few userpages that have had this tag, so that is how I have been handling userpages.
The same thing with old pages, if they have outdated information I tag them in the category of needs some love, so people who want to wiki garden can help give some life to those pages. We have quite a few wiki gardeners who enjoy helping out and I am glad to offer assistance to anyone who needs help with that task.
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There is no automated way to do them and to there are a number of case where I
am not sure we would like to.
I do not see the harm in keeping old user pages.
Outdated content can be updated/adjusted as needed, being old isn't a valid
reason to delete a page though (imho).