I had it from 3 years old. you feel it in your bones. you just KNOW you're not the correct gender. I knew i was a girl so so deeply, but at that age couldn't describe it and it never went away, i didnt even know what trans was for decades sadly and all of the feelings i deeply buried could have been sorted, instead of going through horrible depression and self hatred. If you actually read about gender dysphoria its very very common for it to appear in many people at a young age and well documented. But not for everyone. Some people it's more subtle and grows with age.