Supplement Table 2 Median and IQR for age at first sex and age at marriage by reliability of data and birth cohort for men and women Birth cohort Median (IQR) Men 1950–9 1960–9 1970–9 Women 1950–9 1960–9 1970–9 IQR, interquartile range. Age at first sex All data 20.1 (18.3–22.4) 19.6 (17.9–21.6) 19.0 (17.3–20.9) 19.0 (17.3–20.6) 18.7 (17.0–20.6) 19.0 (17.4–20.8) Reliable data Median (IQR) 19.9 (18.0–22.2) 19.5 (17.8–21.7) 19.0 (17.2–20.9) 18.9 (17.2–20.5) 18.7 (17.0–20.6) 19.0 (17.5–20.8) Age at first marriage All data Median (IQR) 24.5 (22.0–27.0) 24.7 (22.0–27.4) 23.2 (21.2–25.2) 19.1 (17.4–20.8) 18.9 (17.2–20.9) 19.3 (17.7–21.1) Reliable data Median (IQR) 24.6 (21.9–27.0) 25.0 (22.0–27.7) 23.4 (21.3–25.4) 18.9 (17.2–20.7) 18.8 (17.0–20.8) 19.3 (17.7–21.2) A number of non-mutually exclusive classifications were observed for those individuals with three reports (827/3270)— that is, when two reports were consistent and the other report differed by 1 year. In such cases the consistently reported age was used. These criteria were also applied when classifying reports of age at marriage. A detailed description of these classifications, by frequency of reporting, is given by Wringe et al.22 All analyses were carried out using Stata Version 10 (Stata Corp, College Station, Texas, USA). RESULTS The numbers of participants at the first, second and third surveys were 9530, 8210 and 16 266, respectively. In total, 23 517 individuals were interviewed in at least one survey, of which 16 680 were interviewed in only one survey, 3185 were interviewed in two surveys and 3652 were interviewed in all three. Most individuals who were surveyed only once were new entrants at the third survey round in which the eligibility criteria for the study were expanded, as described above. Figure 2 Kaplan-Meier failure curve for age at first sex for men and women. Data include all reports after corrections and estimations for inconsistent reports. i36 Age at first sex All respondents were asked what age they were when they first had sex. Of all the surveyed individuals, 23.2% (5446/23 517) reported never having had sex. Slightly more men than women reported never having had sex (24.3% vs 22.3%, x2 test, p,0.001). Of the individuals who provided a report of age at first sex, 69.3% (12 448/17 953) only reported in one round. A small proportion of individuals did not respond to the question regarding age at first sex (63 (0.66%) in round 1, 36 (0.44%) in round 2 and 48 (0.30%) in round 3). Reports were classed as unreliable for 32.1% of women and 52.2% of men who reported age at first sex in at least two surveys (fig 1). Of these unreliable reports, 31.2%, 21.2% and 15.9% of reports differed by 2, 3 and 4 years, respectively. The remaining 31.6% of reports differed by .4 years. Similar levels of unreliable reporting were observed among only those who participated in all three surveys. The quality of reporting among those in all three surveys was very similar across birth cohorts, as described by Wringe et al.22 Excluding unreliable reports had little effect on the failure curves compared with analysing all reports (regardless of consistency) for both men and women. Median age at first sex remained largely unchanged when unreliable reports were excluded (table 2). Age at first sex was significantly higher for men than for women (log-rank test for equality of survivorship functions: x2 = 110, p,0.001; fig 2). Over all the surveys, 12.6% of men and 12.0% of women reported experiencing first sex at 15 years or younger. Plotting medians by birth cohort indicated that age at first sex declined gradually among men, from 20.7 years for those born before 1955 to 19.1 years for those born after 1975 (fig 3A). For women the median age at first sex remained relatively constant between 19.2 and 19.0 years (fig 3B). The reports from those reporting in at least two surveys were then combined and analysed in a life table format to compare reports by the same individuals as they age. The median age at first sex should remain unchanged as a cohort ages. Among those in older birth cohorts (1965–9 to before 1955), trends in age at first sex remained unchanged as these individuals aged (reading across the table), indicating the absence of reporting biases (table 3). However, among those in younger birth cohorts (1970–4 and after 1975), reported age at first sex increased as they aged. Age at marriage Across all respondents, 36.9% (8676/23 517) reported never having been married. A higher proportion of men than women Sex Transm Infect 2009;85(Suppl I):i34–i40. doi:10.1136/sti.2008.033431