The variable of birth year only available for survey after 1997 in IHIS. However, in original NHIS data, the year of birth is also available for the survey before 1997 (the variable name is birthy). Is there a particular reason that the year of birth is exluded in the IHIS before 1997?
There is no specific reason besides the IPUMS Health Surveys team has yet to include the pre-1997 birth year variable in the IPUMS data extract system. I will send a note to the IPUMS Health Surveys team about user interest in this variable. In the meantime, you can estimate birth year by subtracting the respondent’s age (which is available in pre-1997 samples) from the sample year. This is an inexact method, but commonly used with survey data.