A link please.
Depends on what it is. If it is a style then it will not work with the latest version. Many extensions of that date will work with the latest version. Not sure about bbCodes as there was a major change for phpBB 3.2 - some (many?) may not work.
It is a bit of an anomaly in that styles change for several reasons with each version and one of them is that new events are added so that new extensions will work. Obviously there are bug fixes and new features added.
and the same is true of style authors. many of them are updated/upgraded with each version update as well but it make take some time.
Not necessarily, generally (not etched in stone) style versions must match the phpBB version, many (most?) 3.1 extensions will work with 3.2, and, if you're not sure, ask the author. Custom BBCodes are a different matter but if they work they work, if not there are support forums for approved BBCodes like this one