so when do we become adults?

i'm studying for my developmental psych class tonight and one of the sections of the material is on emerging adulthood. they define this as the period between late teens and mid-to-late twenties when individuals are not adolescents but not fully adults. the authors look to different things in our culture that have been used as a marker for the start of adulthood. here's what they came up with; these things have are what some people think kickstarts adulthood.

college degree.
ability to drink.
ability to vote.
romantic involvement.
child and spouse.
independence from parent.
formation of identity and independence.

i don't think that these exactly hit the mark. i have my own opinions but let me know what you think. what qualifies a person as no longer a young adult? why the period of adolescence?