Overall, Millennials didn’t change how they lived to account for things like climate change. They didn’t stand together as a society and reduce waste.
First, when has ANY age group coordinated across every nationality around the globe to “stand together”? Add to that, the age at which you’re suggesting this should have occurred is right when they were reaching adulthood and barely grasping how the adult world works themselves. At the same time they were coming of age during back to back global financial crises with and even book-ended by one of the most deadly pandemics in human history, while also having the fewest resources to affect change. “Standing together as a society” is unrealistic at any age, much less the one they were given.
Second, I’d argue at the individual level they made some of the largest shifts to low and lower carbon options than any other group. One example would be Veganism which would address the 17% of climate change gases from livestock. Many of them embraced bicycles instead of cars and small EVs such as scooters.
I’m not a Millennial or a vegan nor do I own a small EV scooter, but looking at all the generations alive today, I’d argue Millennials have done the most to combat climate change in spite of the other generations alive working against their efforts.