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How old are young people when they start having sex? Unravelling the applicability of Cox proportional hazards regression

Abstract

Purpose

How old are young people when they start having sex? Although this question is important for educators and policymakers, its answer can easily mislead due to methodology that does not take into account age-censoring and generational trends. This study investigated whether 1-year birth-year cohorts can be jointly modeled by Cox proportional hazards regression to estimate sexual debut ages and to investigate age trends for different sexual behaviors.

Method

We used pooled data from three Dutch periodic population-based surveys, for which 33,377 participants, born between 1980 and 2004, completed an online questionnaire.

Results

Cox proportional hazards regression is appropriate if hazard changes over birth years appear proportionally similar among ages. The Dutch data shows such hazard changes for petting, oral sex, and sexual intercourse, but not for masturbation, French kissing, or manual sex. For sexual intercourse, the Cox model estimates can be interpreted as an approximate linear increase of 1 month in the median sexual debut age per birth-year cohort, resulting in an estimated debut age of 18.17 years in the current (2020) generation of Dutch adolescents.

Conclusion

If the assumptions are met, we recommend using the Cox regression modeling approach to estimate how old young people are when they start having sex, since this method yields precise and current debut ages by pooling information across birth-year cohorts without arbitrarily combining them in multiple-year groups. The age of first intercourse is increasing, so continuous sexuality education throughout high school is advisable, in addition to early sexuality education.

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Acknowledgements

Fifteen Municipal Health Services (GGDs) made a major contribution to the recruitment of a large database containing national as well as regional data.

Funding

This research was supported by a grant from the Dutch National Institute for Public Health and the Environment (RIVM), the Netherlands (grant number 323846).

Author information

Authors and Affiliations

Authors

Contributions

All authors contributed to the study conception and design. Hanneke de Graaf coordinated the study. Judith ter Schure performed the statistical analysis and participated in the interpretation of the results. Geneviève van Liere participated in data collection. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Hanneke de Graaf.

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Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

The MREC UMC Utrecht confirmed that the Medical Research Involving Human Subjects Act (WMO) does not apply to Sex under the age of 25, and therefore an official approval of this study by the MREC UMC Utrecht is not required under the WMO (reference number WAG/mb/16/013562).

Informed consent

For in-school youth, school boards provided active informed consent for participation of their students. In-school students and their parents provided passive informed assent and consent, respectively, where they were able to refuse voluntary participation. Out-of-school youth (≥17 years) provided active informed consent.

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de Graaf, H., ter Schure, J. & van Liere, G.A.F.S. How old are young people when they start having sex? Unravelling the applicability of Cox proportional hazards regression. J Public Health (Berl.) 30, 1873–1880 (2022). https://doi.org/10.1007/s10389-021-01619-6

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  • DOI: https://doi.org/10.1007/s10389-021-01619-6

Keywords

  • Population study
  • Sexual debut
  • Cox regression
  • Adolescence