Well culture changes, someone from the 19th century has a different culture to someone from today. Yes I accept the fact that some arbitrary date isnt that useful, and some (most?) people wont fit the pattern, but I suppose you could say that about any stereotype.

Nevertheless it is a useful shorthand for people going through the same life experiences at the same time.

I say this as someone who's technically a millennial, but old enough to remember when we were generation Y, doesn't eat Avocado toast, didn't have particular problems affording a house and don't particularly like facebook/snapchat/grindr or whatever the kids are on now. So I'm not really a representative Millennial.