New York has raised the minimum age to buy cigarettes to 21.
The law was signed in November, shortly before former mayor Michael Bloomberg finished his second term, and had a six-month waiting period before it came into effect on Sunday.
But its impact is already being clearly felt. "Under 21, no tobacco," warned a small sign at the entrance of a small shop in the North of Little Italy neighbourhood in Manhattan.
No tobacco, either, for anyone who can't present a valid ID proving their age. Shopkeepers scan IDs to test their authenticity before handing over the box of cigarettes.
The measure - unprecedented among America's big cities - raises the legal age to buy cigarettes from 18. It also applies to other forms of tobacco and to e-cigarettes.
It's the latest of New York's efforts to reduce smoking in the city, which bans cigarettes and, as of April 29, e-cigarettes in restaurants and bars, parks and squares and at the city's public beaches.
Some private residential buildings have also banned smoking. Cigarette taxes in the city are also the highest in the country at US$5.85 a pack, which brings the overall price to around US$12.
In addition, the city has established a minimum price of US$10.50 a pack for cigarettes.
Nataleigh Kohn, 23, who works at a start-up company, accepted her ID check with good grace. "It is a good thing. People in high school can't start smoking," she said.
Shopkeeper Muhammad Arisur Khaman said he'd seen some complaints since the law was implemented, but not many.
He just tells unhappy clients: "It's the law."
The higher minimum age was "a step in the right direction", said Pat Bonadies, a teacher walking with a group of students in Union Square.
The 52-year-old said there had been a sea change in attitudes towards smoking. "When I was younger, smoking was much more prevalent among teenagers and pre-teens in restaurants and social settings," she said. "My mother's friends even smoked during their pregnancies."
The city has seen a sharp drop in adult smokers, from 21.5 per cent in 2002 to 14.8 per cent in 2011, official statistics show.
But the smoking rate among young people has been steady since 2007, at 8.5 per cent, which was part of the impetus for raising the minimum age.
The authorities hope that the new law will cut the smoking rate among 18- to 20-year-olds by more than half.