It's a hard question to answer. I feel like I've always known.
I started seeing people as animals when I was around 3 or 4 years old.
My first memory is when I brought something beautiful to show to others and they destroyed it for no reason.
Society likes to say that people grow up but I never really noticed a change since then.
Children aren't "immature", they are just much more honest.
Throughout my childhood I kept seeing how people bully the ugly and the weak, and how my family would always lie about anything as long as it made their day 1% easier. I also saw how my parents had completely different standards for my luckier brother.
I started thinking about it in words around 14 I think, but it didn't change my worldview at all because I was already used to it.
I still think there are good, open-minded people but they are rare and you have to be careful, because most people are only nice while they are in a position of weakness or when it takes no effort.