It's the transition of all transitions: the move from adolescence to adulthood.
Many millennials reportedly said financial independence could be seen as a marker of adulthood. However, for millennials, the concept of adulthood seems to be a feeling, a tipping point, a moment suspended in time that's clear only to the individual experiencing it. Therefore, 20-somethings' transitions to adulthood may be far less concrete than an 18th birthday, a lump sum in a bank account or a pile of stamped envelopes headed to electric companies and Verizon.
Over half of millennials say even though they're past age 18 (the age when their parents likely shipped out of their mom's and dad's place for good), the legal age of independence doesn't turn the "I am now an adult" switch on.
So what makes a person feel like an adult? Is it paying a cell phone bill? Having less time to party? Graduating from college? Getting off your folks' health insurance? A first (somewhat decent paying) job? Getting married?
Truthfully, age 30 feels pretty adult.