Bloomberg rejected criticism from some retailers that the measures would hurt the economy and lead to job losses.
“This is an issue of whether we are going to kill people,” Bloomberg said.
The city estimates that about 27,000 New Yorkers ages 18 to 20 smoke.
Teenagers can still possess tobacco legally. Kids 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 neighborhoods.
Tobacco companies and some retailers had said the age increase would simply drive teenagers to the black market.