the best way to answer this question is to read the paragraph out loud--try to get an understanding for the author's tone. the first sentence is by fsr the most grabbing. the second and third sentences are giving some plain information (who qualifies as baby boomers or millenials). the fourth sentence is merely a lead-in for the next paragraph.
the first sentence has an affect; the other three don't have so much of one. "for the first time in decades" is something that interests you, because it signifies a big change, then the use of the dash before "the millenials" creates a dramatic pause. the emphasis is another thing that usually appears in a hook.