Question about AdAge's classification of Generation Y

In reading the current issue of Advertising Age I got stuck on a description of Gen Y in the article "The 6 Markets You Need to Know Now" by Kenneth Gronbach.  The description of Gen Y that is given is:

"Anyone born from 1985 to the present falls into Generation Y. More than 90 million strong, they've surpassed boomers in size. They are consuming at 500% of the rate of their boomer parents in adjusted dollars, age for age, when you take into account their unprecedented influence on family purchases."

I disagree. Generations are shaped by economic and cultural forces that unify us in fundamental ways. The code of each generation is shaped by that zeitgeist - the glue.  A span of over 20 years is simply too long to sustain a unifying code.

Gen Y ends in 2001. Post-9/11 kids are clearly different and are growing up in a distinct way.  They are shaped by forces like political and cultural anxiety (they will be far more skeptical than Gen Y), post-dot com/web 2.0 technology environment (they will use the Internet differently than Gen Y - hell they probably won't distinguish Internet from any other experience), they are more likely to grow up in cities than ever before etc...

They are also more likely to be children of Gen X - while Gen Y is more likely to be the children of Boomers.  This is key in seeing patterns and developing strategies. I don't want to be too harsh on the article - but this happens too often in developing brand and media plans.  The desire to lump in to large groups and see unity too quickly - when the real answers are much more nuanced.