Centaur's may be bringing a fair comment about flightdeck culture overall, but to suggest bullying is limited to trainers, and more specifically modern trainers, is utterly wrong. Bullying in the cockpit has reduced drastically over the years, but where it persists, is not specific to trainers.
What I think has changed is the reduction in life experience and thus the ability of very young FOs to cope with jibing and banter. I saw this first hand in another loco, where most of the FOs whined about some of the captains (and one TRE and one line shag more than most). There was nothing wrong with the captains; it was the FOs with very large but very fragile egos who simply could not take a well intended joke and crumple under the slightest comment That seems to be an issue with a large proportion, though far from all, cadets. The older FOs, including cadets, tend to have better life experience to equip them with a more appropriate sense of humour and balance of confidence and humility. I feel that it strongly supports a minimum age policy of 25, though Lubitz was older than that. If Lubitz had been older when he started, maybe he would have been better equipped to handle the pressures of flying and his health would not have suffered. That is, of course, speculative.