Cases of teenage sex should be considered on a case-by-case basis and not necessarily reported to the police, government guidance for England says.
Some say the new guidance still leaves room for confusion
But in cases of pre-teen sex, the police should be involved in a discussion with the relevant child protection specialists.
Children's campaigners say children may not ask for advice if they fear being reported to the authorities.
But ministers say the guidance ensures child protection is the first priority.
The guidance, from the Department for Education and Skills, says sex with a pre-teen is a serious matter under the Sexual Offences Act, because a child under 13 cannot legally consent to sex.
There would be cause to suspect the child would suffer "significant harm".
The guidance also says decisions taken by professionals who come into contact with children - including teachers and health professionals - must be fully documented, including where a decision has been taken not to share information.
Cases of consensual sexual relationships between teenagers aged 13 to 15 may be "less serious" than with pre-teens, the guidance goes on, but consideration should still be given to a referral to children's social care.
Cases of concern should be discussed with child protection specialists and "subsequently other agencies if required".
"Where confidentiality needs to be preserved, a discussion can still take place as long as it does not identify the child," the guidance continues.
Professionals are given a checklist to assess the risk to the child, which includes taking account of age, behaviour and maturity.
But the Children's Commissioner, Professor Al Aynesly-Green, said he was concerned the guidance was unclear about when information would be shared.
"Although young people under the age of 13 engaging in sexual activity must always give cause for concern there may be an even greater threat to the safety and welfare of a young girl if she feels unable to seek professional help and advice on, for example, abusive relationships, contraception, pregnancy or sexual transmitted diseases," he said.
The chairman of the British Medical Association's ethics committee, Dr Michael Wilks, said he was "concerned by the lack of clarity in some areas".
But he was pleased the government had decided not to introduce mandatory reporting of underage child sexual activity.
'Not taken seriously'
The Bichard Inquiry, after the murders of Soham schoolgirls Holly Wells and Jessica Chapman, had recommended police be informed where an under-16 was having sex with an older partner.
It said social services "should, other than in exceptional circumstances, notify the police about sexual offences committed, or suspected, against children".
And Sir Michael Bichard expressed concerns that underage sex was not taken sufficiently seriously by social services and the police.
The government's new guidance, Working Together to Safeguard Children, replaces previous guidance issued in 1999.
The Children's Minister, Beverley Hughes, said it made clear that protection of children was always the first priority.
"We know what can happen when vital information isn't shared and acted on where necessary," she said.
"This is a common thread in child protection reports from the deaths of Maria Colwell to Victoria Climbie and the Bichard Inquiry."
She said the guidance "strikes the right balance between respecting confidentiality and empowering professionals to act where a child is at risk of serious harm".