Birth cohort Before 1955 1955–9 1960–4 1965–9 1970–4 After 1975 Men 15–19 19.5 (55) 22.6 (50) 21.0 (313) 24.2 (42) 23.0 (441) 22.0 (244) 23.5 (45) 23.7 (324) 23.0 (307) 21.0 (22) 23.0 (364) 23.3 (216) 23.8 (22) 23.0 (254) 23.0 (234) 45–49 24.0 (219) 23.0 (168) Women 15–19 17.0 (357) 18.0 (59) 18.0 (963) Table 4 Median age at marriage and number reporting (N) for men and women reporting in at least two surveys by 5-year birth cohort and age group 20–24 25–29 30–34 35–39 40–44 20–24 25–29 30–34 35–39 18.0 (67) 19.0 (710) 18.3 (542) 18.0 (63) 18.7 (645) 18.7 (573) 19.0 (49) 18.0 (803) 18.3 (492) Supplement 40–44 19.0 (41) 18.0 (661) 18.0 (664) 45–49 18.7 (425) 18.0 (541) Data include all reports after corrections and estimations for inconsistent reports. Small sample sizes are indicated in italic, along the leftmost diagonal for men and for women. These are low due to the timing of the surveys in relation to birth cohorts (eg, the majority of individuals in the 1970–4 birth cohort had aged past the 20–24 age group when they were surveyed). reported age at first sex, one may expect decreasing social desirability bias and increasing recall bias as an individual ages. Such effects may generate the similar levels of unreliable reporting observed across all birth cohorts. We previously reported delays in sexual debut among teenage men and women in the first 3 years of follow-up.12 In this earlier analysis we compared trends in the percentage of individuals reporting having had sex within a small population subgroup who were starting sex at that time (17–19-year-old men and 15– 17-year-old women). These age restrictions were chosen to analyse an entirely new group of maturing adolescents at each survey, thus focusing on very recent trends. These previous findings, using current status data only, are therefore not directly comparable to the longer-term trends in median age at first sex in the entire population presented here by birth cohorts spanning a period of .40 years. In Zimbabwe, information on timing of sexual debut has been collected in four DHS surveys since 1988, and median age at first sex has been calculated by province for the most recent three surveys.23–26 In Manicaland, median ages among women aged 25–49 years are relatively constant at 18.3, 18.2 and 18.4 years in 1994, 1999 and 2005–6, respectively. Among men aged 25–54 years (25–49 years in the 2005–6 survey) the median age at first sex increased from 19.5 years in 1994 to 22.0 years in 1999 and subsequently decreased to 20.6 years in 2005–6. Thus, the medians calculated for women in this study population are a good representation of those in the wider population in the Manicaland province whereas, for men, the medians calculated for this study population are generally younger. Our estimates of median age at first sex are relatively high in comparison with other African countries such as Kenya, Tanzania, Uganda and Zambia where median age at first sex ranges from 16.4 to 18.1 years.21 The substantial gender differential in age at marriage observed here is not surprising as women are expected to marry younger than men in Shona society.27 Women may seek to marry at a relatively young age in order to obtain the security associated with marriage and subsequent motherhood. Men are expected to obtain employment in order to afford bridewealth, thus re-enforcing the expectation that they wait until reaching their mid 20s before marrying. Recall bias may influence quality of reporting of age at marriage, particularly among older individuals. In this society, marriage is a long process and individuals may be unable to distinguish their age when the Sex Transm Infect 2009;85(Suppl I):i34–i40. doi:10.1136/sti.2008.033431 actual ceremony took place from that when bridewealth was paid, from when the relationship begun. The biases identified in reporting of age at marriage may also be partially attributable to the change in the definition of a long-term relationship. A mathematical modelling study parameterised for this population has shown that the effect of delaying age at first sex on subsequent risk of HIV infection depends on the length of time spent single (between first sex and marriage).13 When sexually active and unmarried, the risk of infection is double that when married (2% and 1% per person-year at risk, respectively).13 Thus, the extended duration of time spent sexually active and single among men, and the brief duration of time spent sexually active and single among women observed here, support previous findings in this population that, in general, young men acquire HIV from premarital relations (as well as extramarital relations) whereas, for women (who typically spend much less time single), the strongest determi- nants of infection are their spouse’s behaviours.28 29 Previous cross-sectional analyses (using the first round of data) investigating age at first sex in relation to subsequent risk of HIV infection found that, among both men and women in this population, riskier sexual behaviour was observed among those reporting an early age at first sex (younger than 18 years) than in those who reported an older age of sexual debut.3 However, for men, an early age at first sex which coincided with age at marriage did not lead to subsequent riskier behaviour compared with men who initiated sex at an older age. This suggests that, among men, early marriage may be somewhat protective against the riskier effects of early age at first sex. Take-home messages c High levels of reports of age at first sex and age at marriage were identified as unreliable among men and women attending multiple surveys. c Excluding unreliable reports did not alter the observed trends in age at first sex and age at marriage. c Comparing birth cohorts over a 40-year time span, median age at first sex has remained constant over time among women but has declined gradually among men. c Median age at marriage has remained relatively stable for both men and women. i39