It was a gradual and very personal internal decision, that happened over time. I started being concerned about what "meat" actually is when I was five or six, and I stopped eating other mammals when I was about 28.
I was working in a fine-dining restaurant on the Outer Banks, and every time I went into the kitchen to pick up orders, I saw a squirming lobster being thrown into boiling water. I felt ill, and that was part of my finally really facing my own feelings about another creature suffering and dying, just because a human finds it tasty.
That is my simple answer on a subject that has complexities even to me, one who has been rescuing bugs from drowning ever since I could walk. ;)
I stopped eating meat when I was 18 and went out on my own. I followed 'Mary Tyler Moore's' advice. I don't eat anything that has a face.
Hamburgers don't have faces
My experience happened when was 2. I was in my high chair and my mom was feeding me chicken pot pie. Suddenly, I had this awareness that "chicken" and "meat" were living animals that had been killed for us to eat. I realized these were chunks of dead bodies. I was done eating meat instantly. By the time I was 4, my motto was " I will not eat anything that has or has had a face." In the years since, I have added eggs,honey,and gelatin as well...among other animal derived items.
I stopped eating meat in my teens and then began eating it again on a fierce craving many years later. I was an athlete. That's one of my reasons.
I stop eating meat when there's nothing left on the bone or when the buffet closes.