Baby boomers have long been considered the generation that did not want to grow up, perpetual adolescents even as they become eligible for Social Security. Now, a growing body of research shows that the real Peter Pans are not the boomers, but the generations that have followed. For many, by choice or circumstance, independence no longer begins at 21.

From the Obama administration’s new rule that allows children up to age 26 to remain on their parents’ health insurance to the large increase in the number of women older than 35 who have become first-time mothers, social scientists say young adulthood has undergone a profound shift.

People between 20 and 34 are taking longer to finish their educations, establish themselves in careers, marry, have children and become financially independent, said Frank F. Furstenberg, who leads the MacArthur Foundation Research Network on Transitions to Adulthood, a team of scholars who have been studying this transformation.

“A new period of life is emerging in which young people are no longer adolescents but not yet adults,” Mr. Furstenberg said.