Defining or describing a "hipster" is a lot like what Justice Potter Stewart said about pornography: "I know it when I see it".
Not all hipsters are this obvious about it. And it's not an age thing; I've known people whom one would have to call hipsters who were teenagers, and others who were in their seventies. As Rationalist noted: "...it's less to do with the lifestyle and more to do with the pretentiousness." I have a turntable and I still buy the odd record, but I don't look down at people who don't; the same with second-hand shops and silent films. I don't define myself by what I like in clothes, entertainment, etc., nor do I judge others for liking a different set of things. The "hipsters" I've known tend to smirk a lot, and feeling superior and above it all is their default setting.