Almost my sentiments.
17 years old is a "young adult" in my book. 18 and older (and graduated from HS) is an adult, in my book. All behavior aside, that is.
I had this talk with my son at age 13...I consider "teens" between the age of 13 and 17 to be "young adults". They're not "children" any more, but they're not yet full grown adults, either. They're in that stage of their life where they're actively growing into adulthood, pushing their limits and working towards independence.
The key, I informed my son, is to realize this: if you want to be treated as an adult, with all that goes along with it, then ACT like an adult. If you want to be treated like a child, then act like a child...but remember that a child cannot do anything on their own without supervision, nor can they be trusted by themselves.
He was also informed that, while he was under my care until 18 and graduated and that I make SOME allowances for "backsliding", once he turned 18 he's an adult in the eyes of the law. This means he will be held accountable as an adult for the consequences of his behavior, childish or otherwise.
17-22 is MORE than old enough to be exhibiting adult behavior, most especially around firearms.