explaining

When are they old enough to start explaining about consent? My four-year-old boy to my two-year-old boy: "I don't want you kissing me right now. If someone doesn't want you to kiss them it's not nice to kiss them even though kissing and hugging is a nice thing. So don't kiss me. Okay?" To which the two-year-old replies: "Oooooooooh!" as though he had never heard this before!