I'll often find answers to questions which are outdated and haven't been improved. I agree they have a mechanism in place to fix it, but are users actually fixing answers en masse? SO could introduce an age out mechanism that requires an answer to be maintained semi regularly? Do they already have this today?
Editing an answer substantially upsets the entire up/down vote system, as well as the comments. A better system would be something like question/answer versioning. In general it's a problem that has received far too little effort.
Do you really believe that adding a tag with a version number to a post is something that can be classified as a very aggressive move, particilarly if it's an old post?
Yes, it really is. Tags are used to filter results. Stackoverflow specifically lets users run searches only on specific tags, or even exclude certain tags from the search results. As tags reflect technologies and their versions, stackoverflow enables users to run queries only on specific versions.
99% of people use Google as the search interface to stack overflow.
As someone who primarily interacts with SO via search engine results, it's one of the biggest problems with SO. The number of top voted/accepted answers that are 'wrong' in one way or another keeps going up and thus the chance of me finding the 'right' answer on SO keeps going down.
Otherwise, how is it "not a big problem"? Is it the sort of thing where "in theory" it's not a big problem, even though in actuality it significantly effects the usefulness of the site to many people?
That's not a big problem. Questions and comments can be edited by other users, which includes adding and altering tags that reflect software versions.