# Trends in Median Age at First Sex and Age at First Marriage Among Youth in Tanzania: Accelerated Failure Time Model (1994–2016)

## TL;DR

The study finds that the median age at first sex and first marriage in Tanzania increased over time, with education playing a key role in delaying these events.

## Contribution

The study introduces the use of the accelerated failure time model to more accurately assess trends in age at first sex and marriage among youth in Tanzania.

## Key findings

- Median age at first sex increased by one year for both sexes between 1994–2016.
- Education is associated with higher median ages at first sex and first marriage.
- The AFT model detected gradual increases that traditional statistics missed.

## Abstract

Understanding trends in age of first sex and first marriage is vital for interventions addressing sexually transmitted infections (STIs/HIV) and youth sexual behavior. Shifts in these milestones affect fertility, contraceptive use, and STI/HIV dynamics. Traditional descriptive statistics often overlook younger populations, leading to inaccurate trend assessments. This study analyzed trends in median age at first sex and first marriage using survival analysis. Data from eight surveys within Magu Health and Demographic Surveillance System (1994–2016) were analyzed, focusing on individuals aged 15–24 years. The accelerated failure time (AFT) model with log-logistic distribution estimated these medians. Results showed an increase in median age at first sex by one year for both sexes and in first marriage by one year for females and two years for males. The AFT model captured gradual increases from 2003–2004 to 2015–2016 for females and 2003–2004 to 2010 for males, while standard descriptive statistics showed no changes in specific periods: age at first sex, 1996–1997 to 2012–2013 (females) and 2003–2004 to 2012–2013 (males); first marriage, 1994–1995 to 2015–2016 (females) and 2003–2004 to 2010 (males). Individuals with no education had lower median age at first sex (males: 17.1 vs. 18.3 years; females: 16.2 vs. 18.2) and first marriage (females: 18.0 vs. 21.3) than those with secondary or higher education. HIV-positive individuals experienced slightly earlier age at first sex and first marriage than negative individuals. Education plays a pivotal role in delaying these events. The AFT model enriches trend assessment.

The online version contains supplementary material available at 10.1007/s10508-025-03282-4.