The National Prosecuting Authority has taken on parliament - and one of South Africa's highest courts - over their decision to lower the age at which children may agree to sex.

State counsel FC Roberts slammed the Supreme Court of Appeal for slashing a convicted paedophile's sentence because he had sexual intercourse with his teenage victim after the boy turned 16.

Pretoria dentist Izak Andreas Geldenhuys is hoping that the Constitutional Court will confirm the Appeal Court's finding that the difference in the age of consent for boys and girls was "unconstitutional", resulting in six of the abuse charges against him being dropped.

Geldenhuys, who is 28 years older than his teenage victim, will then serve only four years in jail for his abuse of the boy.

Roberts argues that the uniform age of consent should be 18, despite parliament's decision to set the age of consent at 16. The age of consent for same-sex sexual acts had been 19.

In documents before the Concourt, Roberts claims courts were "ill-equipped" to decide on a uniform age of sexual consent for children.

He further raises doubts over whether the Appeal Court's ruling served the best interests of children.

According to Roberts, it was "bizarre" that nobody was allowed to possess pornography in which a person of 16 or 17 appeared, or show pornography to 16 or 17-year-olds, but could legally have sex with a 16- or 17-year-old.

The NPA had raised these aspects, he said, "to dispel the impression created in the judgment of the Supreme Court of Appeal that setting the age of consent at 16 years is a simple matter".

Evidence before the Pretoria Regional Court, where Geldenhuys was originally convicted on 13 charges of indecent assault and sentenced to 11 years in jail for abusing the teenager and his younger brother, earlier revealed that he had a long history of child abuse.

In 1991, while working as a dentist in Balfour in Mpumalanga, he pleaded guilty to four counts of indecent assault and received a suspended three-year sentence.

Geldenhuys then took a job as a bus driver, through which he met the boys he was convicted of abusing.

He told the regional court evidence of abuse found on his teenage victim's body could have come from a "relationship" the boy had with another boy.

The Appeal Court was not convinced, but Judge Belinda van Heerden said the state had failed to prove the boy had not consented to sex.

This meant the sexual acts performed by Geldenhuys were statutory, rather than criminal, offences.

Supported by four other judges, Judge Van Heerden then found that the distinction in the age of consent between boys and girls contained in the Sexual Offences Act of 1957 - under which Geldenhuys was sentenced in 2005 - amounted to unfair discrimination, asking the Constitutional Court to confirm this ruling.

The hearing is scheduled for August 28.