Yes 17 is a child.. that is if you are still living at home, still dependent on your family for food, a roof over your head...
If you are living in your own place, playing rent, earning a living and doing everything for yourself (money, laundry, food, rent) then I would consider you an adult.
I have a 16 , 19, 20 and 21 yr old. I consider the 21 yr old an adult..JUST ..because she is away from home for most of the year and is not dependent on me. The others are still dependent.. QED still children in all ways that actually matter.
Age does not make an adult. Maturity does.