Suppose 30-year-olds make exactly the same as their parents did at 30. Then we would expect that barely half of 30-year-olds earn more than their parents did at 30. This title is stupid. It implies "oh hur dur the economy is so bad" when in reality it says nothing.
Economic growth is not 0. So, really it says that 1/2 the population is flat our worse off. Further, a rising tide lifts all boats is a complitly false analogy, and anyone using that phrase to talk about the economy is lying.
The numbers are in real USD, not nominal. So in reality, it's a huge fallacy to say that we're so much worse off that in the 1970s. We're almost exactly the same as the 1970s.
The main difference to point out here is that the 1970s 30-year-olds made much more real USD than their parents' generation, while 30-year-olds today are making almost exactly the same. Just because income isn't growing as much doesn't mean the 30-year-olds today are worse off.