Tobacco sales will be banned to under-21s in New York in a new health drive by mayor Michael Bloomberg.
The law will make it the first large US city or state to prohibit sales to young adults.
City health officials hope that raising the legal purchase age from 18 to 21 will lead to a big decline in smoking rates in a critical age group. Most smokers get addicted to cigarettes before 21, and then have trouble quitting.
The ban has limitations, in terms of its ability to stop young people from picking up the deadly habit. Teenagers can still possess tobacco legally. Children will still be able to steal cigarettes from their parents, take them from friends or buy them from the black-market dealers who are common in many neighbourhoods.
But City Health Commissioner Thomas Farley said the idea was to make it more inconvenient for young people to get started, especially teenagers who had previously had easy access to cigarettes through slightly older peers.
"Right now, an 18-year-old can buy for a 16-year-old," he said. Once the law takes effect, in 180 days, he said, that 16-year-old would "have to find someone in college or out in the workforce."
Tobacco companies and some retailers had opposed the age increase, saying it would simply drive teenagers to the city's thriving black market.
"What are you really accomplishing? It's not like they are going to quit smoking. Why? Because there are so many other places they can buy cigarettes," said Jim Calvin, president of the New York Association of Convenience Stores. "Every 18-year-old who walks out of a convenience store is just going to go to the guy in the white van on the corner."
Mr Bloomberg also introduced changes that will seek to keep the price of tobacco high by prohibiting coupons and other discounts and setting a minimum cigarette price of 10.50 dollars (£6.52) per pack.
Large cigarette companies now commonly offer merchants incentives to run price promotions to bring in new customers.
"For someone who might be trying to quit smoking, it makes it easy for them to buy on impulse," said Mr Farley.
Mr Calvin said the elimination of discounts would just further feed the drift away from legal cigarettes, and toward illicit supplies brought into the city by dealers who buy them at greatly reduced prices in other states, where tobacco taxes are low.
Both bills were passed by the City Council late last month. The legislation also prohibits the sale of small cigars in packages of less than 20 and increases penalties for retailers that violate sales regulations.