Hi Theodore! Thanks for your interest in the IPUMS API!
Annual data are actually a bit tricky. NHGIS includes American Community Survey (ACS) data back to about 2009. Assuming you are familiar with how ACS data are packaged, we have 5-year data for 2009 (covering 2005-2009) through 2017 (covering 2013-2017). We also have 1-year data for 2010 through 2017 and 3-year data for 2010 through 2013. If you’d like more clarification on ACS data packaging, let us know. All of these data are available through the API (look for datasets including “ACS”).
For years prior to 2009 we currently only have general demographic data for decennial years. BUT, the good news is that we are very close to releasing annual tract estimates of population by age and sex for earlier years. These estimates are based on the Census’ Annual Population Estimates. The NHGIS annual tract estimates will initially be available simply as files for download off the static site (nhgis.org, look under “Other Resources”). They will eventually be incorporated into the main NHGIS database, at which point they will become available via the API.