At What Age Must a Boy or Girl Observe this Touch Prohibition?

Hello, Today on this page—Ask the Rabbi—there is a question about from what age the prohibition of touch applies. The rabbi wrote that a man is forbidden to touch a girl from the age of three, and a woman is forbidden to touch a boy from the age of nine. But from what age are they considered a man and a woman upon whom this prohibition applies? Thank you and all the best.