The first thing that came to my mind were gazetteers, but none of the digital/data-based once I looked at included dates. 

There are a few book gazetteers I found that appear to have founding date information:

Historical Gazetteer of the United States by Paul T. Hellmann (not comprehensive)

American places dictionary : a guide to 45,000 populated places, natural features, and other places in the United States ... in four volumes by Abate, Frank R. 

Omni gazetteer of the United States of America, by Abate, Frank S

The researcher mentioned a Census dataset that costs money.

Does anyone know of something like this that's available in dataset form?

Many thanks!

Susan