Final answer:
Financial independence is the most important marker of the transition from adolescence to adulthood according to young people.
Step-by-step explanation:
The most important marker of the transition from adolescence to adulthood according to young people from their early teens to their late 20s is financial independence. While completing formal education, marriage, and moving out of the parental home are also significant milestones, financial independence is often viewed as a key indicator of adulthood as it signifies the ability to support oneself and make independent decisions.