At my house it has been a gradual process for several years. When kids turn 18, they technically are adults, but in reality they are still just kids. When DD turned 18, she ended up going to her first year of college in town here. For the most part, she was still financially dependent on me. Year 2 found her moving down to AZ for school since she didn't like it at the college here. She took out a student loan in her name with a co-signer, and started assuming responsibility for her living expenses and tuition down there. I still carried her on my health insurance for part of that year, although second semester she signed up for insurance through the university. Up until this month (she just finished year three of college), I still had her on the family plan for her cell phone and was paying her auto insurance. However, she is taking a year off of school in order to work and establish residency. So that is why she has now taken over her cell phone and auto insurance. She is no longer financially dependent on me at all. So at 21, I would say that she is actually becoming an adult.
I never charged room and board, because a.) she is still family b.) she was only staying here for the summer, although last year and this year she has stayed down in AZ and c.) she wasn't making much money. I guess I would feel differently if she were not going to school and not contributing to work around the house etc. So I guess the final answer is that there is no set age for becoming an adult IMHO. It is a process rather than a specific milestone.
