Today I learned that 28-year-olds aren't adults.
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.
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 much as adjusting their wants to be more in line with what is realistic-- choosing one or half of one of the above listed items.
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.
I fully believe you can be an adult without any of the above.