Really interesting. I suspect there's something about the political climate more than the age of the city that dictates the orientation of the roads. Where societies where strongly controlled by small group of leaders (religious leaders, monarchs, city planners, etc) the streets ended up much more coherent. Where cities grew organically as needed the layouts are less coherent. Also, what happened in Charlotte?!
When a city is 1000 years old or more, the societies that control it will inevitably shift between more and less strongly controlled over the years, so unless it suffered a massive razing that required full reconstruction not long ago, there's only so much a central planner can do.
You can see that in Lisbon, for example; the downtown area is quite grid-like since it was rebuilt by a single leader after the earthquake and subsequent tsunami of 1755, but the rest either stood or was built later, under a variety of different regimes - the Roman Empire, Visigoths, Caliphates, Christian monarchies, etc.