This is purely anecdotal, but my experience is that there isn't so much a generational correlation to work ethic as there is an age-based one. Which is to say, a person's work ethic can vary significantly over the course of their adult life.
What I've seen in my own workplace is new entrants (whether at graduate or non-graduate level) entering the organisation but generally (and of course there are outliers) without making a full commitment to it and, particularly in the case of graduate entrants, trying to keep their student lifestyle running for a few more years. Over time, usually by the late 20s, this transitions into a much stronger work ethic; more time spent in the office and more "commitment" to the organisation. Eventually, somewhere usually in the 50s, this lapses into a degree of burnout. Now, all of the above is a huge generalisation and based on personal experiences only, but I've seen a couple of generations go through that cycle now.
Of course, there's a far bigger correlation between work ethic and social class. Behaviours liked to worth ethic, such as the ability to focus on deferred reward have a strong hereditary component, whether based on biology, culture or both. Again, there are exceptions, but this is where I've seen the strongest correlation. In the mid-2000s (at the height of the UK's New Labour touchy-feely period) I worked for an employer which took part in a Government-subsidised scheme to give placements to "disadvantaged" young people. This was actually a pretty cushy detail; the pay for those brought in through the scheme wasn't huge, but it was significantly above the minimum wage (almost Â£10/hour) and the work was white-collar administrative. Moreover, there was an expectation in place that if you did well, you would be able to turn it into a full-time job (this was in the land of silly-money before the big crash, when the UK economy appeared to be in full boom). Hell, there wasn't even much of a dress code beyond "use your common sense and don't wear anything that would actively harm our reputation".
I was involved with this scheme for three consecutive years, once as a mentor and twice as one of the "lucky" managers "given" one of the apprentices (yes, there was a degree of corporate arm-twisting). Across all three years, with an intake of 8-10 people per year, not a single one stuck with it for more than 2 months. The simple basics of being expected to get into the office at a sane time (we had a flexitime-within-reason system), to come into the office every working day and to follow the instructions of a manager once in the office were too much for the participants.