I disagree, it's a common theme that they have no sense of loyalty to a company and keep hopping around..
And what's wrong with staying at one company for so long? I've been at the same company for 20 years and have no desire to leave yet. Why should I leave after 45 years?
Actually in this day and age with a large percentage of companies only offering short to medium term contracts, sometimes job hopping is what a person needs to do to keep food on the table.
Furthermore, IME very few companies promote from within these days. Myself and most of my friends climb the ladder by looking to external opportunities.
Unlike my Dad who has had 3 jobs in his life, with his current job been over 25 years (different roles, but always within the same org), I'm up to my 7th (not including the 3 years I worked for myself), and yet I'm only in my early 30's. Do I think that's a lot? Not really, in that time I've taken a redundancy and had 3 jobs which where short term contacts (3 months a piece, that said I spent several years on 3 month contract after 3 month contract and the money allowed me to buy a house, so I can't complain), so one could argue that my previous companies have shown me loyalty, so I am hardly about to do the same the other way.
All that said, I am in my current gig the longest I've ever been in a position (a little over 2.5 years) and since my goals and my companies goals are presently aligned I see no reason to change jobs again.
Now say what you want of Gen Y, but assuming that I had stayed in my first role for the last 12 years, (assuming I hadn't gotten the redundancy), based on wage policies the company had at the time I would be earning half what I am now (and I would probably know a quarter of what I know now as well, and I probably wouldn't have done anywhere near as much traveling). Seriously who can blame me for taking opportunities when they arise?
So look, say what you like about Gen Y's willingness to go out and find opportunity and take risks, personally I wear that as a badge of distinction on my sleeve that we're not willing to simply take cough (but then complain about it into our beers at the end of the day) from those higher up, and that since society wants to hand us lemons, we're willing to make lemonade with them.