Could masturbation cause infertility at the age of 13?

Of course not: If it did, the human species would have died out long ago. The claim that enjoying your body as all male and most female teens do will cause infertility isn't true, and you should be angry about being lied to. Sperm count goes down if ejaculation is more frequent simply because there is more fluid available to dilute them. Especially, embrace your emerging ability to love wisely ; well.