I am trying to estimate the total population over age 55 in cities with only 50,000 population or less. Which city varaibles do I need to use to identify those cities and group responses by those cities?
IPUMS NHGIS contains data on the number of residents, broken down by age group, for a number of different geographic levels. Which geographic level you use depends partly on how you are defining cities. When you refer to City, you are most likely interested in Places. NHGIS has information on Places and Consolidated Cities from all recent censuses, and the ACS. Places essentially consist of consolidated cities and towns, plus other named areas with a significant concentration of population, but which are not officially incorporated in the state. I’ll refer you to the Census Bureau’s Geographic Areas Reference Manual for more details on these. You can find a list of all current Census Designated Places here.
I think the tables you’ll want are “B01003. Total Population” and “B01001. Sex by Age”. These are both available broken down by Place and by Consolidated City.
If you haven’t use NHGIS before, you might find this video tutorial on using the NHGIS Data Finder helpful.