Older generations consider the gen Y age group to be entitled, spoilt, lacking commitment, and generally impatient with regards to earning their place in the world.
When you combine this with the "new startup" - cheap, glamorous, very low barrier to entry for the technically adept, then it's easy to see where the conflict comes from. Twenty-somethings see the perfect opportunity to jump into the booming tech startup area by themselves where they have a huge advantage - being both born with the web and often still in the safety net of college or the very early stages of their career. Older generations never had this type of opportunity, and simply don't think the younger people have deserved or earned it.
Personally I think that there is no better time to do your own thing and be innovative than your early twenties, and that conventional workplaces eventually erode the type of skills needed for a startup; of course, then there is the other issue - if building a startup is like jumping off a cliff and making a parachute on the way down, is someone still a "real" entrepreneur if there is a safety net at the bottom?