When is it time to let go?

At 17, if you've done a good job, the kid should be ready to start exercising some adult responsibility.  Mine at that age were allowed to work, open bank accounts, and were held responsible for discretionary buying.  I also let them take the car out alone, and didn't interrogate them when they came home. I did, however, encourage them to talk about their day - did that from the time they were about six or so - and luckily there were no surprises.  Since they are also on the threshhold of college, consulting them, rather than telling them, about career choices, choice of college and the like establishes that you see them as mature enough to start making their own decisions.  Actually, this process should start before they reach 17, that way the stress of letting go is lessened.