Scarleteen

I've heard that if you have too much sex at a young age, it can cause infertility.. Is that true?

Infertility is most often associated with untreated sexually transmitted infections, complications from previous pregnancies or deliveries, or congenital issues.

So, if the sex, be it a little or a lot, someone had when young was unprotected and they also did not get regular STI testing and treatment, then yes, that could be an issue. But without any of the above factors, sex all by itself has no impact on trouble with fertility, no matter someone's age.