Teenage smoking rates in England have dropped since the legal age for buying cigarettes rose from 16 to 18, research by University College London suggests.
Researchers surveyed more than 1,000 teenagers aged 16 and 17 before and after the age rise in October 2007.
Smoking rates in this age group dropped significantly, from 24% before the law change to about 17% after it. Smoking rates in over 18s remained unchanged.
Cancer Research UK said it showed tobacco policy could make a difference.
Experts know that more than 80% of smokers start before the age of 19 and that half of all long-term smokers will die of cancer or other smoking-related diseases.
Stopping young people from starting smoking is vital if the death toll from tobacco is to be reduced, they say in the journal Addiction.
Jenny Fidler, who led the study and is based at Cancer Research UK's health behaviour research centre at University College London, said: "The new law looks to have helped reduce smoking prevalence among younger age groups.
"This is good news for the future health of this generation of young people and shows that tobacco policies can make a real difference."
Before 2007 it was legal to sell cigarettes to anyone over the age of 16 in England, Scotland or Wales. The same was true in Northern Ireland until 2008.
Jean King, Cancer Research UK's director of tobacco control, said: "This result is encouraging. We would like to have seen an even bigger drop in the number of young smokers but any measure that helps stops young people from smoking is a step in the right direction.
"We need to do more to protect young people. We urge the government to prevent more lives being lost to an addiction that will kill half of all long term smokers."
She said putting tobacco out of sight in shops and removing cigarette vending machines would be a good place to start.
A Department of Health spokesman said they were in discussions across government on how best to progress to tackle smoking.
He added: "We welcome these findings as nearly all adults who smoke get hooked when they are young. Smoking is the biggest preventable cause of death in England, causing over 80,000 premature deaths in England each year."