I spent many of my childhood years on a farm... So I knew it from an early age...

I was born into a vegetarian family. So I was taught early where meat came from.
Also we had a few cows in our yard, so there was no confusion about the source of milk.
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What interested me about your question is something I read about Dog Psychology way back.
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Dogs became the best friends of man when they found humans were better hunters than them.
By being friendly to humans, dogs found they could live a comfortable life without a care about food.
They helped humans hunt down animals and take away the meat and were happy to eat the leftovers such as bones, intestines and sinews sticking to the bones.
Dogs considered humans the best hunters ever to stalk the wilds. Dogs were all appreciation for the human capabilities where hunting was concerned.
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Long after dogs lost their significance as helpers in hunting, humans continue to keep dogs as pets. Dogs too have not forgotten the past help humans gave them in finding food.
So the dogs remain loyal, respectful, loving and obedient to humans to this day.
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Today when a dog sees its master coming home after shopping, it assumes that the master is returning after a hunting trip. The Dog assumes that the master has mastered the art of hunting without the dog's help. The dog knows the master returning home would have brought enough food for the family and the dog. It wags its tail in appreciation.
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So the Dog assumes the food from the supermarket is really something that the master got by hunting in the jungles.
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There are plenty of humans too who think the food from the supermarket is actually manufactured goods and not taken from killed animals.
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I think the dog psychology and human psychology match in some respects.

I grew up in farm country that got converted into housing and a high school in my teens. I can't remember when I became aware of what I was eating. It seemed like I always knew that eating meat meant something had to die.

Very early, I grew up in farm country and was babysat by the neighboring farmers. They raised black angus and limosine cattle - steak.

It wasn't until third grade that I truly realized the horror of animal treatment that is acceptable to most who simply don't care. I became less selfish, and am proud to say have been a vegetarian ever since!

The first house I lived in was on a farm, only for the first 3 years of my life but I was never lied to about where meat, milk & eggs came from. I watched my father slaughter sheep, milk cows & collect eggs from the hen house. The slaughtering part of it is what scared me away from eating meat...

It was the other way around. I saw the hog slaughtered and the meat hanging in the smoke house before I ever went to a grocery store.

I live next to a cattle mart in a farming community so I have always known I think. A survey done in London though, showed that 1 in 5 primary school childern when asked where does milk come from said Tesco!!(Its a big supermarket)

A very early age. I knew ham and bacon were from pigs, steak from cows...

My uncle ran a butcher shop and had a farm. I don't remember ever not knowing. I did think bones were made of wood until junior high, though.