rather vague
http://en.wikipedia.org/wiki/Millennialsbut on the whole millenials corresponds to generation Y. According to sources birth dates are 1978-1994 or 1980s to early 2000s.
Four or five generations - a generation is as kashew says, around 20 years. Now that makes 100 years at least. I wonder if it is indeed 4 or 5 age groups. What I understand for "plus de la moitié": "of the four or five generations working together, over half of them will be millennials." To be clearer, "of the people comprising the 4 or 5 generations, over half will be millenials"