Is the cutoff for age in the ACS 1st of April?

I am using the ACS (IPUMS-USA) for several years and would like to specify date of birth as precisely as possible. For the variable age the description is the following: “AGE reports the person’s age in years as of the last birthday.” What exactly does “last birthday” mean, is it age as of 1st of April in the year of the survey?

If the cutoff-date is indeed 1st of April, then an individual whose age is reported as 10 years in the ACS 2010 was born in 1990 if his quarter of birth is the 1st quarter. In case the quarter of birth is the 2nd, 3rd or 4th quarter, that individual was born in 1989.

Can anybody confirm or reject this? Thanks a lot!

Your suspicion is correct. Since age is defined as the age as of last birthday and BIRTHYR is derived by subtracting AGE from the survey year, then this calculation is one year late for individuals who have a birthday after the survey date. The BIRTHQTR variable can be used to refine the calculation of BIRTHYR for all respondents in the 1960-1980 IPUMS-USA samples, and for people under age 1 in 1870, 1880 (excluding the 100% database), and 1950 samples. In all other samples, where no additional information on the date of birth is available, it will be off by one year for everyone who has a birthday after the survey date. BIRTHQTR is also available in the 2005-onward ACS/PRCS; however, since the date of survey is not fixed as it is for the decennial censuses, the true year of birth cannot be determined. Because ACS/PRCS respondents are surveyed throughout the year, though, the errors averaged across the entire sample will be close to zero.