There are rumours that, at 14, Daniel Radcliffe is "too old" to play Harry Potter any longer. Quite different speculation surrounds his female co-star, Emma Watson, who at the same age is an "elegant young woman"

There are rumours that, at 14, Daniel Radcliffe is "too old" to play Harry Potter any longer. Quite different speculation surrounds his female co-star, Emma Watson, who at the same age is an "elegant young woman" who would choose 40-year-old Brad Pitt as her "love interest" if she could. While Daniel is already heading for the scrapheap, future red carpets beckon for Emma.

Why is it that girls find it so much easier to make the transition into adult acting? Christina Ricci (left) is a former child star who may have a few answers. She has had little visible difficulty in maintaining her successful acting career into adult life. Or as The Sun (sometime scourge of paedophiles) put it this week, as it published pictures of Ms Ricci in her underwear: "My, who's a big girl now?"

And Ms Ricci is not the only one. Loads of female child stars, whether they take off their kit for men's magazines or not, have little difficulty moving into adult roles. Male child stars have a lot more difficulty achieving adult success. Those who do - like Mickey Rooney, Kurt Russell or Elijah Wood - are as pretty as girls anyway. Their brand of masculinity is seldom very powerful.

And it's not just about pretty young women selling sex, although that's part of it, of course. There's an element in this disparity which illustrates powerfully our different expectations of the two genders. Womanhood is seen as the goal and the aim of girlhood, with metamorphosis from girl to woman encouraged and fetishised. (Young woman to old woman, of course, is a good deal more problematic.)

Manhood is seen as more threatening. There's a real undertow of belief that men are rough and frightening creatures, who have little connection to the cute little people that they once were. For both sexes, the stereotype is damaging. But it helps to explain why boys find it harder growing up than girls do, while woman find it harder growing old than men do.