That's plain wrong, they have less disposable income and lower net worth than the two generations before them had at the same stage in their life.
Just because stock market grew doesn't mean life of an average Joe improved.
This stuff is complicated so make sure you’re comparing like with like. Even the chart above ignored one critical adjustment: in 1970 just 5% of the population was foreign born, while today its 15%. Millennials are much more likely today to be recent economic migrants, or the children of them, than boomers. That drags the numbers down, but doesn’t imply anything negative about millennials’ economic status. Indeed, those immigrants are experiencing upward economic mobility, and are seeing themselves that way. The dour attitude among millennials seems to be mainly on the native born, who really are doing as well or better than their parents.
At least, they’re doing better economically. Thanks to social changes, they have less agency than their parents did. They have few opportunities to garner authority and respect in their communities. They’re not building families and getting the hope that comes from nurturing the next generation. They have many reasons to be dour. Their parents failed them!
Culture, meanwhile, is the organically developed heuristics and norms of communities.
You can measure the relative worth of these things by looking at the functioning of society on various metrics. Unlike say civil engineers, practitioners of *studies and social sciences have demonstrated little ability to come into a community and make live measurably better for everyone. Arguably, as traditional social norms have been eroded, many communities are less healthy than ever—much more so than you’d expect looking at just the economic numbers. Millennials as a generation are the richest ever, yet seem profoundly unhappy and unsatisfied.