Many questions on StackOverflow relate to the current status of a particular language, component, or software tool (current version.) The majority of questions over 18? months old can be called into question as they are outdated.
It's highly likely that a new version of the specific language, component, or software tool has been released since the time of the question and the answer may no longer apply. Often you obtain the wrong answer on StackOverflow as the question is old and the missing/broken functionality has been addressed since that time.
Tags could be more version-dependant as a softening agent but it's a generic problem of all of these sites. A new user of Stack Overflow could easily come to the site with a particular question and be provided with an answer that was voted up and selected as the best answer only to find out later that it's no longer the case. Or, worse, they may never find out later that the answer has changed and they help contribute to the ongoing fud about a product/tool...
All answers are only considered best at the time of the response. All answers are subject to scrutiny over time. In the least, "old" answers could be highlighted differently to warn the user of the length of time the answers were provided. It's definitely debatable what constitute "old" information so perhaps that would be configurable by tag moderators for lack of a better idea.