You make excellent points and I appreciate you sharing them. As a matter of fact, I agree with most of your comments. Overprotection can cause major problems for your child including rebellion later in life. Especially if they go away to college(i.e. drinking, excessive partying, promiscuous behavior,etc). I just believe that this particular situation calls for being a little overprotective. Playing outside and running around the neighborhood is on another level from being alone in one of the largest metropolitan cities in the world.
Yup. As I've said from the start, this situation is a bit extreme for most people, but I believe that for kids who grow up in the city, 9 years old may not be too much of a stretch for an individual kid who is particularly mature.
I grew up in suburban South Jersey, but my parents both grew up in cities (my Dad in Philadelphia, my Mom in Camden, NJ). As a child, I went to a Catholic elementary school that was across town from us - about 5 miles away. Nearly all of my childhood friends were from the area right around the school - so from about the age of 9 or 10, I was allowed to ride my bike across town to play with them. I had to cross a major highway (Rt. 70 in Cherry Hill for anyone familiar with the area) and several large secondary roads, but I had been taught how to do it safely and was responsible.
By the age of 14 or 15, I was allowed to take the PATCO train into Philadelphia with friends to go do things. We went to a 4th of July concert on Ben Franklin parkway - nearly a million people there - with just 3 of us and no adults. We would often take the PATCO train, and change to the broad street subway to go to Phillies games.
We were suburban kids, but were raised by urban parents who were not afraid of the city.