Hannah only turned eight a couple of months ago – but she has already started puberty. Her mother, Liz, is shocked. "Over the last few weeks her nipples have suddenly become swollen. They're really painful for her – she finds it hard to put on a T-shirt or to hug people. We went to the doctor and he said it was hormonal changes linked to the onset of puberty. I was horrified. Hannah's a skinny little thing and I hadn't anticipated puberty for at least three more years."
Yet puberty is arriving earlier and earlier. Last week it was reported boys' choirs were finding it hard to maintain their standards because they now lose experienced choristers much earlier – trebles who used to sing until the age of 15 or 16 are now having to stop at 12 or 13, because their voices are breaking.
Peter Hindmarsh, professor of paediatric endocrinology at Great Ormond Street Hospital in London, says it's harder to collect data on male puberty, and that reports of puberty starting earlier in boys are anecdotal. But for girls, the evidence is conclusive.
"Between 1958 and 2005, breast development – the sign that puberty is starting in girls – moved about nine months ahead of where it used to be," he says. "Interestingly, though, the age at menarche (first period) hasn't moved much – from about 13 to about 12.8."