It really is not as black and white as you are trying to make out.
You don't magically turn into an adult overnight when you reach a specific age.
Yes, an average 17yr olds is more mature than an average 15yr old. but adult? No, I don't think so.
An adult has generally completed their initial education.
An adult has an awareness of life including most of, but not limited to
- living alone, or without older people at least
- budgetting
- holding down a job
- understanding politics, legal issues such as wills etc
- understanding that 17 is not an adult
- organising own holidays, and saving up for them
By this list I was not fully adult until I was 23 or so.
Instead of all these separate posts, why don't you post what is really bothering you (ie show you are mature enough to do that), and you might get more insightful answers.
e.g.
I'm really fed up. My Mum insists on knowing when i'll be home, and doesn't like my BF staying overnight. I'm upset because I've been held back a year, and have no friends. i'm self harming (because my step dad abused me when I was 12 and I feel guilty about it) and the school told my Mum. We are arguing all the time. I'll be an adult next month when I'm 17 and wish they would all leave me alone. I want to move out and live on my own (I'm sure I'll get benefits to help me). How can i explain all this?
Say what's really bugging you and you may get some help.