As less young people meet major economic milestones such as moving out and getting married, the concept of adulthood is changing.

Fewer young adults are achieving economic and family milestones typically associated with adulthood, according to a recent working paper from the U.S. Census Bureau.

According to the working paper, “Changes in Milestones of Adulthood,” almost half of all young adults in 1975 had reached four milestones associated with adulthood: moving out of one’s parents’ home, getting a job, getting married and having a child.

Five decades on, that progression has changed dramatically. The share of young adults that have followed the traditional pathway to adulthood has dropped to less than a quarter, according to the paper.