I could add something more apropos physical threats and death threats online for public positions I've taken on sexual violence. That is absolutely accurate.
To speak quickly to how early we can start, it's from birth. My children are in the CPE system in Quebec, and they are taught not only to ask permission to touch others but also that they are allowed to say no when somebody touches their own bodies. The notion has been very much inculcated in them that their bodies are their own and that other than when they have to go to the doctor or I have to take their temperature, they have veto power over their bodies. I see that reflected in the way my kids comport themselves now. They are much more respectful of their friends' bodies and theirs and mine. They will now ask if they can hug me. They've learned to do that because I am also a rape survivor, so that's been important in establishing boundaries in our household.
Children are capable of understanding consent. They're capable of understanding their own bodily autonomy. There's nothing wrong with teaching them to be respectful and to seek consent from the get-go.