The NLSY79 Child and Young Adult data files include several distinct child age variables at each survey point. The most appropriate age variable to use may depend on one's research objectives. The age variables found in the two Areas of Interest called CHILD BACKGROUND (for all children) and in YA COMMON KEY VARIABLES (for Young Adults) reflect consistency checks and a small number of edits based on additional information. All the created child age variables in the CHILD BACKGROUND area of interest are computed in months, regardless of whether they refer to the Child sample or the Young Adult sample. The codebook, however, displays these "age in months" variables in grouped ranges for ease of viewing. The "age of child at mother's interview date" variables (AGECH) that are assigned to the CHILD BACKGROUND area of interest are created for all children (including young adults), regardless of age. Also, children who are currently young adults are still represented in the variables that indicate child age at assessment date for the survey rounds when they were actually assessed. These child assessment age variables describe the age of the (now) young adult at the time s/he was assessed in prior survey rounds. Generally, most young adults will only have assessment age data for years when they were under the age of 15; however, in the early Child survey years before the start of the Young Adult data collection, some young adults were assessed even though they were older than 14.

The variable that describes the mother's age at the birth of the child (MAGEBIR) is computed in years as is the mother's age at interview date (AGEMOM) which is in the FAMILY BACKGROUND area of interest. The ages for young adult children, available in the Young Adult areas of interest, are also computed in years (e.g., AGE1B assigned to YA FERTILITY AND RELATIONSHIP DATA - CREATED, and AGEDEC, AGEINT in YA COMMON KEY VARIABLES). Users are encouraged to rely on the created age variables rather than any age variables that are direct pick-ups from the questionnaires in the various survey rounds.

Child age variables

There are three relevant age variables for younger children for 1986 through 2014, with AGECHyyyy and MSAGEyyyy also available in 2016. In most but not all instances, the values for these age variables will be the same in any one survey year. Unlike the age variables for the young adults, Child age variables are computed in months, so users who prefer a variable in which the unit is in "years" will need to apply formats or perform a simple conversion.

One series of child age variables (AGECH) references the age of the child as of the mother's interview date. Similarly, there is a Child Supplement assessment age (CSAGE) based on the date the interviewer-administered assessments in the Child Supplement were given. Another age variable series (MSAGE) is linked to the date the Mother Supplement was administered. This variable, termed "Child age at mother supplement date," is appropriate to use when one's research utilizes an assessment that was administered as part of the Mother Supplement questionnaire. The child supplement assessment ages and the mother supplement ages are assigned to the CHILD BACKGROUND area of interest.

Which age variable is best?

Users are advised to exercise caution when applying age variables in conjunction with the child assessment data. Some unedited child date of birth and age variables may appear in the CHILD SUPPLEMENT and MOTHER SUPPLEMENT areas of interest. These items, not available for all children, appear exactly as recorded in the field. Users are generally discouraged from using these items as reported directly from the questionnaires and instead are urged to rely on the child age variables found in the CHILD BACKGROUND or ASSESSMENT areas of interest.

Most of the child assessments are designed to be administered to select age groups of children. Since assessment dates are not always the same for the Child Supplement and the Mother Supplement, users should apply the age variable specific to the supplement that was used to administer the particular assessment. In 2000 (and only 2000) this issue became somewhat more complex in that two mother-report assessments (the HOME and Temperament) were administered in the Child CAPI Supplement for children under age 4. In all years except 2000, it is advisable to use the Mother Supplement age at assessment (MSAGE) for the HOME and Temperament scores. In 2000 the Child Supplement age (CSAGE) was a more accurate indication of assessment age of children under age 4 for these two measures.

Young Adult age variables

There are two Young Adult age variables per round most appropriate for use. In contrast with the variables for the younger children, these are computed as age-in-years. One references the Young Adult's age at his or her interview date and is the variable that most people will want to use. The second is the Young Adult's age on the last day of the calendar year for that survey round. This variable is included because December 31 of a given reference year defines the eligibility of a child for inclusion in the young adult sample and drives some of the skip patterns in the survey starting in 2016. From 1994 to 2014, a child who was 15 or older as of that date was included in the young adult sample. This is the reason that some tables in this guide that provide child age as of an interview date split 14-year-olds into a Child group and a Young Adult group. Beginning in 2016, with the Child Supplement and Child Self-Administered Supplement (CSAS) no longer fielded, children aged 12 and over as of December 31 of the target year have been included in the Young Adult sample. Children aged 12 and 13, however, answer far fewer questions – some of which come from the CSAS – than those 14 and older.

The age at end of year variable can also be useful for defining a sample according to an unchanging age cohort as of any interview year. Interview dates are rarely exactly two years apart; therefore, an individual who is age 20 at one interview point, while typically 18 at the preceding interview, might possibly have been 19 or 17.

Age and demographics

A number of child background variables are provided in the child data files that designate each child's date of birth, birth order, sex, and mother's race. This series of variables is updated in each release to reflect information for all children as of the current survey point, including children who have become young adults. These key variables, assigned to the CHILD BACKGROUND area of interest, are updated to incorporate children born since the last interview. The demographic information is also reviewed in light of mother updates from the main Youth file. Included in this series is an indication of the child's usual residence at the time of the mother's survey. From 1979-1981 and in 1983 and 1985 the child's residence status is based on reports from the mother's household roster. In all other years, child residence information is derived from the child-specific questions on "with whom the child usually lives" in the Fertility section of the main Youth questionnaire. The set of variables in CHILD BACKGROUND also includes a variable, FSTYRAFT (C00052.00), that can be used for connecting child events with information linked to the mother's survey date.

Data updates and child age

Information on a child's date of birth from the Children's Record Form (CRF), an instrument used with the main NLSY79 until recent rounds, was the source of birth date information for the Child Supplements. Beginning in 1988, a Child Face Sheet was introduced as an aid to interviewers in the calculation of child ages. This instrument contained a preprinted child birth date or a place for the interviewer to record the child's date of birth from Part A of the CRF and provided a place for calculating child age in reference to the Child Supplement interview date. This paper Face Sheet was replaced in 1994 by a CAPI feature that computed child age so that interviewers could anticipate which assessments would be administered.

A child's birth date may occasionally be altered on the basis of new information received from the mother in conjunction with the internal evaluation procedures carried out at CHRR. Thus, in a small number of cases, date of birth and child age information may not be completely consistent across all survey rounds. Appendix 5: NLSY79 Supplemental Fertility File Documentation, in the NLSY79 Codebook Supplement, discusses cases in which child birth dates were edited.

Over the interview years occasional revisions have been made to a child's date of birth if it was found to be in error. However, the questions and assessments administered to a child were contingent on the child's age as specified at the time. For a variety of reasons, no attempt has been made to alter the historical age record when a date of birth was revised. Thus, when using age-sensitive information from prior interview points, two options are often possible. One option is to recreate an age variable based on the most recent date of birth of child in conjunction with the interview date in that year. (Note that if this changes the child's age, it may no longer mesh appropriately with the age-specific assessment information collected for that child). The second option is to use the set of age variables from that year, a variable that will be consistent with all the other information gathered from the child in that year. In most instances, this latter option is probably the preferred solution. Finally, in almost no instance would it be appropriate to simply decrement a child's age by the number of years between the most recent interview and the interview round of interest. This can often lead to incorrect estimates for the reason noted above.