Why isn't it taught?
When I was young, noone ever told me about the issur of masturbation. By the time I found out, I was deeply entrenched. I was doing it a few times a day. The guilt was overwhelming and the inability to talk to anyone about the big dark secret was torturous. My best years were destroyed by this. Why
Why isn't this taught in yeshivos when boys are 11-12 years old?
I agree it should be addressed earlier. We have a booklet called "Protecting our Youth: Experts Speak" that can help parents know how to speak to their children about these things.
But let me just try to set you straight about something. Your best years weren't destroyed by this. This is a common misconception. The fact that you are still struggling with it AND making progress, that is one of the most defining characteristics of your holy neshama! It is precisely the struggles that we undergo that turn us into who we need to be. Hashem gave Avraham Avinu 10 huge tests. Did he claim that these tests destroyed his best years, or did he understand that these challenges are what enabled him to truly grow and serve Hashem?