My son was about three when he noticed. Then he started telling the whole neighborhood about his parent's private parts.... 'do you have a big weinie like my daddy?" that sort of thing. That's when we started covering up and talked to him about privacy... and what is okay to talk about and what isn't! We also had the whole not letting people touch you in private areas talk and not touching other kids in those areas, and all the info that goes with that.