While the year of demarcation for who is a Millennial depends on the source, I like the years 1978-2001, so anyone born during this period is considered a Millennial in my book. What is more important than who is a Millennial is the laundry list of characteristics and stereotypes used to define this generation. Simple searches on-line will reveal that Millennials are considered, self centered, egocentric, over ambitious, immature, uncouth, and a whole long list of other "flattering" commentary. However, I don't think there are many in the Millennial generation who would deny those hash-tags as a group. They (we) are all of those things and more. They are proficient multi-taskers, they are quick thinkers (or Googlers maybe?), they are tech-literate, and they are uniquely creative in a world not quite ready to accept their creativity. Which is why the culture clash is coming/already happening.
To me the clash will unfold in many ways. Typical to anything where there is a clash between old and new, there will be conflict, there will be compromise, and there will be reformation. This big clash will be followed by smaller conflicts, less compromise, and less reformation until there is complete indoctrination and a total shift in thinking. The history of the world is littered with examples of this type of mind changing clash, the geocentric vs. the heliocentric, or the flat-Earthers vs. the round-Earthers, team Coco vs. team Leno. In all these cases both sides thought they were right, both sides claimed to have supporting evidence and both sides claimed to have support of the masses and their supporters ultimately did battle.
However no matter how hard fought the battle maybe sometimes the outcome is inevitable. Change is going to happen. Leno got his show back, the earth was proven to be round, and the Sun is the center of the solar system... for now...I still say Pluto is a planet! In any event, the main thing to remember if you find yourself set to clash along generational lines is to consider the opposite position in the debate.
In regards to the Millennials, those of older generations should think of and consider the way things are done now in terms of how they will be done in the future. A phone call today may still be important, but tomorrow it may be as useless as a telegraph. So instead of being upset the Millennials prefer electronic communication, think of it as inevitable that virtually all communication will eventually be electronic... until such time it becomes telepathic, come on Steve Jobs, get a move on it already, I want an APP for that!
In regards to dealing with older generations; Millennials, some things are set in stone, respect for others (especially those in authority) is one of them. If you think your boss is wrong, by all means, tell him, privately and respectfully. If he doesn't allow you to, then he is simply a poor manager. However going home and blogging or posting to FB and Twitter about how much of a Dbag your boss is not respectful, its cowardly. That is the behavior that leads to the stereotype of all Millennials being immature. Put his role into perspective, would you want someone posting to the world that you were awful at your job? I doubt it... and before you can say "so what", "it happens" or "get over it" think about how it may not have ever happened to your boss. You may be the first, and if he is like most bosses today, you may also be the last to criticize him on the world wide web.
Ultimately, while mutual perspective is important it won't stop the impending cultural clash, just make it a little less dramatic. The office and internet will become the Haight-Ashbury of the new generation. Too me though, the biggest reason for the inevitability of the clash is that too many people within the existing establishments will take defense of their viewpoint and begin to have pitched battle with the opposition. Let's face it silo mentality is definitely the hallmark of the older generation. The heretics may burn, some Millennials may not survive... but the world of business will change.