I'm sure people who fought in WW2 said that about the boomers, who in turn said it about the gen Xers. In 20 years time we'll be reminiscing about emoji and snap chat and hipsters and whatever the kids are doing now.
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.