You can see this most clearly in the Boston skyline I think. There are two types of building here: historical and 50s retro future. A skyline like this wouldn't be built all at once in a city as big as Boston, so it's safe to assume buildings have been built like this for a while

Now, there are cities like Austin that have relatively architecturally homogeneous skylines, but that's because of maybe growth that took it from a small city to one of the largest in the country over about 20 years. I have a hard time believing that could apply to somewhere like Boston