Hey - a ring! Let's throw a hat in there!
The problem here is that the famous search engine doesn't return the most relevant result first. This tends to be self-perpetuating, since people tend to follow early links first, and so the famous search engine marks those as more popular -- even though realizing that a part of a URL constitutes a version number and giving the more recent one priority isn't hard to do. It is, in fact, urine simple. But as I say, this is a problem with a famous search engine, not with the indexed web site.
I don't agree that telling asking politely the famous web crawler not to index documentation for older versions is a good idea. Larger organizations especially are typically a ways behind the upgrade curve, and you want to be able to find the documentation for your version using the famous search engine rather than the unmitigated horror show local site search the site owner provides.
But some sort of alert on older-version pages seems like a great idea. There should be a link to the most recent version of the same page, if available, or to the lower-most matching node in the documentation tree if not.
Ideally, each page should also contain information on which versions it's relevant for. That way you'd know, if you were looking at an old page, whether there'd be any point in looking at a more recent one.
/Uffe