How can I determine the average year of construction of buildings in a city?
Check the municipal tax records if available, check the toilet tank lid, check the architectural style with Google Earth.
Most cities now have on-line municipal property tax assessment records you can access for most neighbourhoods. This will tell you when most homes were built, aside from in-fill and any new builds. If you check about a dozen and they all fall around the same year, you have it.
Strangely enough, if you live in the neighbourhood you can check the underside of the toilet tank lid for the date the tank was installed - will be close to the time the neighbourhood was built. Furnaces and water tanks may also have a date on them.
You can also check Google Earth street view for the architectual style in the community. This will probably be accurate to the nearest decade.