Parents in Wales have been warned that under-15s should not drink alcohol - even under supervision.
Issuing new guidance, chief medical officer Dr Tony Jewell said alcohol harmed a child's physical development.
"The younger people start drinking, the greater the impact on their health," he said.
Under-age drinking adviser Menna Boynes said the challenge was to accept children would do what they were told not to and to protect them as they did.
The new guidance, You, Your Children and Alcohol, aims to help protect children and young people from the risks linked to alcohol consumption.
The Welsh Assembly Government quotes research which shows that 40% of Welsh 15-year-olds drink alcohol on a weekly basis, with 20% having been drunk for the first time at 13 or under.
A spokesman said a public awareness campaign would begin in July and a video booth was available on the assembly government's stand at the Urdd Eisteddfod to gauge young people's views on alcohol.
The views gathered would help inform the development of campaigns to tackle alcohol misuse in young people.
Dr Jewell said he was "not trying to preach" to people with the new guidance.
"I aim to simply set out the facts to help parents and young people make informed choices," he said. "Ultimately, it is a matter for individuals if they take this advice on board."
He said evidence showed that the younger people started drinking, the greater the impact on their health and well-being in the long term.
"Not only is there a clear immediate and long-term impact on health of alcohol misuse, there is the potential for young people to be at risk of being involved in anti-social behaviour, crime, unwanted pregnancies or drugs," he said.
Dr Jewell said the cost to the NHS in Wales of treating alcohol-related health problems currently stood at between £70-£85m a year.
He warned: "If we do not tackle this problem, the financial cost in the future will be significantly higher."
Assembly Health Minister Edwina Hart said the power to make the most significant change on child consumption of alcohol remained with the UK Government, through legislation on price, licensing and advertising.
"I will work with the UK Government on this but if it does not take action soon, I will seek the powers to act ourselves," she said.
British Medical Association (BMA) Cymru Wales secretary Dr Richard Lewis said: "Children under 15 are still developing their brains, and drinking alcohol can do severe damage and have long-lasting consequences.
"Unless we combat the heavy drinking culture which has developed amongst young people, we are more likely to see increasing numbers of people with serious liver disease at a younger age in years to come."
Andrew Misell from Alcohol Concern said an alcohol-free childhood was "the ideal for all children".
"All of us as adults need to set the right example so that children can learn how to treat alcohol responsibly," he said.
"We also need to ensure children aren't tempted to buy or get hold of cheap alcohol and the easiest way to do this would be to introduce a minimum price for alcohol."
However Ms Boynes, of the Specialist Under-age Drinking and Drugs Service (Sudds), said young people generally knew that they should not be drinking alcohol.
"With the 13-16 age group, particularly, that is the time when children start to want to do what they know they shouldn't do," she said.
"The challenge for services like mine is balancing the fact that they know they should not drink with the knowledge that they are going to do it anyway.
"It's a question of limiting the damage."
Ms Boynes, who works to inform and support young people in Carmarthenshire, Ceredigion and Pembrokeshire on the problems of alcohol and drugs, added: "Longer term consequences mean very little to younger people. It's really difficult to take a long-term view at that age.
"It's about facing the fact that they will have a binge when they are under 15 and giving them the advice that keeps them safe while they are doing it."