When my friends and I were 13 we were all jerking off, but wee secretive about it, and always accusing each other of excessive jerking off.
Five of us boys did everything together. Usually this included things such as swimming, fishing, or hiking. On long hikes in the country the conversations inevitably got around to masturbation. All of us admitted we did it, but no one was willing to acknowledge that he was an addicted excessive masturbator. No one except Alan who, like me, was uncut. When I denied that I did it more than twice a week, we were skinny dipping at a secluded lake. Alan said he masturbated about 20 times a week, and anyone else who did it that much and was uncircumcised would have a red line, as he had, just at the end of his foreskin looking like a drawstring closing the opening. He had one, and so did I. I blushed bright red at this, and the rest of the group howled with laughter at my embarrassment. Alan was right.