For example, in lbpain3mo, the variable definition lists as NIU people above the age of 18. But when I tab lbpain3mo by age in the 2015 data, thousands of people above the age of 18 are listed as NIU. This happens with some of the other pain variables as well. What is the other limiter that is removing these people from the universe?
This observation is likely due to the fact that you are tabulating variables such as lbpain3mo for all members of the household 18+ as opposed to tabulating the variables for the sample adult from the household.
NHIS underwent a major survey redesign in 1997. After 1997, interviewers continued to collect information on the household, socio-demographic characteristics, and basic indicators of health status, disability, and utilization of health care services for all persons. However, to reduce interview length and biases from proxy reporting, a new sampling scheme for the Basic Module questionnaire was adopted, in which more extensive information was collected on one randomly selected sample adult and one sample child from each family.
Therefore, if you would like to tabulate variables such as lbpain3mo for the sample adult in the household, you will want to use the variable ASTATFLG to identify the record of a sample adult.