She got to know the couple after the girl was placed on a beyond parental control order. The girl's mother also made a police report against the boy.

The girl was the oldest of three children and was always out of the house. She had sex with the boy and others too.

Said Dr Balhetchet: 'They just looked like normal 12-year-olds. Previously, the girl might be more well-endowed or the boy might be bigger, but it isn't the case any more.'

While children physically mature only upon puberty - on average at age 12 for girls and between 13 and 14 for boys - children today may exhibit sexual behaviour in early childhood.

'There are extremes of children between four and six years old who exhibit sex play because they have been exposed to media which glamorises sex,' said Dr Cheng Tai Kin, a paediatrician at Kinder Clinic in East Shore Hospital.

'This kind of socialisation is becoming more common, so sex play, which normally emerges at a more physically and emotionally mature age, can occur in very young children.'

Sex play involves children exhibiting sexual behaviour or pretending to have sex.

It is also becoming more common for younger teenagers to engage in sex, he said. Two years ago, he saw a case of a pregnant 13-year-old. 'She was just a normal child,' he said.

Experts say sexual encounters tend to take place in the home when the youngsters are alone.

Among two youth close in age, sex is usually the result of boredom or curiosity. It often occurs among children with family problems or latch-key kids, said Dr Winslow.

Dr Balhetchet noted that in cases where one teen is much older than the other, there may be some mental or emotional coercion.

Experts said pre-teens and teens may develop sexually transmitted infections or become pregnant as they are less likely to use protection.

They may also start believing that they are grown-up and independent and may even run away from home, said Ms Sheena.

Dr Warren Lee, a paediatric endocrinologist in private practice and a senior consultant at KK Women's and Children's Hospital, said children these days tend to develop sexually before they are emotionally mature.

Ultimately, said experts, the responsibility must lie with parents.

Said Dr Lee: 'It is up to parents to encourage responsible development of the mind and soul even as children are physically developing. If a child is physically 12, parents should ensure that he is also 12 years old emotionally.'

Added Ms Sheena: 'Children are exposed to so many elements like the media and school, there is no way to avoid it. Parents can help through supervision, by being involved in their lives and knowing what they are up to.'

This article was first published in The Straits Times on Dec 14, 2008.