does that mean ALL 17 year olds?
at 17, I was moved out, in college and definitely an adult.
eta: and in today's day and age, kids are growing up much faster.
Yes, it means all 17 yr olds. Just as at 17 a person has not completed the physical growth and development of their bodies, and there is still bone growth that will happen among other things, the human brain has not fully developed, and the hormones that make it function to give us the ability to see long range consequences from actions and curb impulsivity have simply not developed yet.
Check it out on Web MD if you like, it is a physical and psychological fact that the law and the Courts recognize.