The law, signed November 19 shortly before former mayor Michael Bloomberg finished his second term, had a six-month waiting period before it came into effect -- but its impact can already be clearly felt.
"Under 21, no tobacco," warned a small sign at the entrance of a small shop that sells smokes, newspapers, candy, coffee and cakes, in the Nolita neighbourhood (North of Little Italy).
No tobacco, either, for anyone who can't present a valid ID proving their age. Shopkeeper scan IDs to test their authenticity before handing over the box of 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, in parks or squares, and at the city's public beaches. Some private residential buildings have also banned smoking.
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Cigarette taxes in the city are also the highest in the country: USD 5.85 a carton, which brings the overall price to around USD 12. In addition, the city has established a minimum price of USD 10.50 a box for cigarettes.
"It is a good thing. People in high school can't start smoking," she said.
Thomas Wall, 24, a former smoker who works in architecture, agreed, though he said the measure probably wouldn't eliminate teen smoking all together.
He compared the new age restriction to the ones around alcohol, which set the US drinking at at 21.
When underage people want alcoholic drinks, they often get them from older people who buy for them.
Shopkeeper Muhammad Arisur Khaman said he's seen some complaints since the law was implemented, but not many. He just tells unhappy clients: "It's the law."