Earlier quoted context omitted.
I'm curious if that sentence is itself an explanation for the perception that a 28 year old might not be an adult. In other words, at least in American culture, adulthood is strongly tied with stable employment. If the 15 - 29 age bracket in general does not enjoy stable employment, that could itself be responsible for creating the perception that members of this bracket are not adults.
Adulthood is also tied with marriage, home ownership, car ownership, child-rearing, etc... these things have been pushed back a decade or so as a result of too little money. By the standards of the 1950s, the 20-somethings of today aren't adults. That has sociological twists to it as well, of course-- but primarily economic. I suspect that people aren't actively avoiding having a house, car, wedding, or children, so…
No. Just no.
Those are things which some adults do, that does not mean you aren't an adult if you don't do them.
Is a 40-year-old with a successful career who just never got married not an adult?
Stop insulting and marginalizing people.