If your platform offers tagging, use it. Tag by version and by feature (if the post is about a particular feature) and by platform if you support more than one. (Even if you intend them to be the same, your Windows implementation is probably a little different from your Linux one.)
Versions: If you're doing frequent releases you probably have major and minor versions. For example, Firefox pushes new versions all the time, but things don't really change much from 37.0.3 to 37.0.4. There might be bigger changes when 38.0.0 comes out, though. I recommend a tag for each major version. Stack Overflow, for example, has separate tags for Java 5, Java 6, Java 7, and Java 8.
Category tags: But sometimes the version doesn't matter; your question is about (to continue this example) Java. Or Firefox. Or your product. So in addition to the version tags, have the category tags. (This is assuming that your site covers more than one category/product.) If your site supports hierarchical tags then make the category tag the parent of the specific-version tags.
Features: Most of your questions aren't going to be about the product as a whole; they're going to be about specific functional areas. Whatever those are in your product, create tags for those. If the boundaries are fuzzy, analyze a sample of your existing questions and figure out how they should have been tagged. Then do it.
Platforms: Probably helps; won't hurt. Someday you're going to release a Windows-only plugin or a Linux-only console or...something.
I've described a general approach to taxonomy here, but how does this help you with your problem of identifying old content? First, version tags are pretty obvious here; when you're no longer supporting version 6 and even your most stubborn users have moved on to 7 or 8, any question tagged 6 or earlier is subject to review. But don't freeze or delete them just yet! Some of those questions might still be relevant in later releases; a question way back on Firefox 3 that's about local caching might still be relevant, while one about the URL bar probably isn't. You can use the feature tags for additional filtering here.
The feature tags themselves can help you. Suppose you introduced a shiny new widget several versions back and later removed it (your users hated it or whatever). Questions about that widget will be obsolete when everybody has moved on to the newer version.
As for platforms, pretty soon you can probably retire those questions that were specific to Windows XP.
As for what to do about it, there's rarely value in outright deleting content (that isn't or wasn't wrong) just because it applied to an older version -- so long as it's properly tagged so that no reader would mistake it for current. Somebody might still be using that, or be tracing something back through several versions ("what did this feature replace, anyway?"), or just be curious, or have followed a link from some other site. Or be helping Uncle Bob, who never moved on from his Windows 95 machine with the CRT monitor but who still wants to browse the web... ever so slowly. You just never know when information will still be useful to somebody.
You might, however, consider closing old posts to further edits, in part so you're not spending effort maintaining it and in part to signal that, really, this isn't current. Some sites (like Stack Exchange) have the concept of the "historical lock" for posts that are now obsolete but are heavily linked or otherwise still interesting.