The mayor also signed companion legislation setting a minimum price for all cigarettes sold in the city at $10.50 per pack. Andrew Burton/Getty Images

Mayor Michael Bloomberg signed landmark legislation Tuesday banning the sale of tobacco products to anyone under the age of 21, making New York the first large city or state in the country to prohibit sales to young adults.

During a brief ceremony at City Hall, Bloomberg said raising the legal purchase age from 18 to 21 will help prevent young people from experimenting with tobacco at the age when they are most likely to become addicted. City health officials say 80 percent of smokers start before age 21.

The mayor, a former smoker, also signed companion legislation setting a minimum price for all cigarettes sold in the city to $10.50 per pack. That law also bans retailers from offering coupons, 2-for-1 specials or discounts.

In signing the bills, Bloomberg deflected criticism from some retailers that the measures would prove economically harmful and lead to job losses.

"This is an issue of whether we are going to kill people," Bloomberg said. People who raise the economic argument, he said, "really ought to look in the mirror and be ashamed."

The ban has limitations in its ability to stop young people from picking up the deadly habit. Teenagers can still possess tobacco legally. Kids will still be able to steal cigarettes from their parents, bum them from friends or buy them from the black-market dealers who are common in many neighborhoods.

But City Health Commissioner Thomas Farley said the idea is to make it more inconvenient for young people to get started, especially young teens 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, Farley said, that 16-year-old would "have to find someone in college or out in the workforce."