The large majority of smokers start smoking before age 21. According to the National Survey on Drug Use and Health, over 80% of adult smokers smoked their first cigarette before they turned 18, and nearly 95% started before age 21.[1] Raising the minimum legal sale age to 21, along with proven tobacco control strategies, could prevent youth tobacco use.

What is the Minimum Legal Sale Age?

The minimum legal sale age (MLSA) prohibits retailers from selling tobacco products to anyone under that age. In most places across the country, that age is set at 18. Four states (Alaska, Alabama, New Jersey, and Utah) have a minimum age of 19. However, historical news archives and tobacco industry documents reveal this was not always the case. In 1920, at least one third of states had a MLSA of 21, but those laws were eroded in part due to aggressive tobacco industry lobbying.[2] Now, momentum is building across the country to raise the MLSA to 21 to prevent more youth from initiating tobacco use.
In 2005, Needham, Massachusetts was the first town in the US to enact a law raising the MLSA to 21. As a result, smoking rates decreased by 47%, three times as much in the four years following as rates in towns surrounding Needham. [34] Since then, over 100 Massachusetts towns and counties have done the same, including Boston.
New York City made news in October, 2013 when they passed historic tobacco control legislation, including raising the MLSA to 21. Many communities nationwide have now followed suit, including Boston, MA, Cleveland, OH, Chicago, IL, and many localities in the Greater Kansas City Area. As of March 2016, the total list includes over 220 cities and counties in 16 states.  
In June 2015, Hawaii became the first state to raise the MSLA to 21, effective January 1, 2016. California became the second state to do so on May 4, 2016. State-wide initiatives have also been proposed across the country including in Connecticut, Illinois,  Iowa,  Kentucky,  Massachusetts,  New York,  New Jersey, Oklahoma, OregonPennsylvaniaRhode Island, Tennessee, TexasUtahVermontWashington, and West VirginiaFederal legislation to raise the minimum legal sale age for tobacco products to 21 was introduced on September 30, 2015. 
The idea enjoys broad public support from 75% of nationally surveyed adults, including from current smokers and from individuals aged 18-21, who would be most impacted by the law. [56] Public support is consistent across all regions of the US and is higher than support for raising the age to 19 or 20 years. [7] The policy has also been endorsed by the American Academy of Pediatrics.

Sales Age vs. Purchase Age

One challenge for implementation within the Tobacco 21 movement has been around enforcement. One approach is to place the burden of responsibility on the tobacco retailer rather than on the underage purchaser. For example, in March 2012, the City of Chicago voted to raise the MLSA for tobacco to 21. However, the ordinance also removed the fine for possession of tobacco for those underage, holding the tobacco seller accountable rather than young people who have been targeted by the tobacco industry.
Some cities have included provisions that penalize tobacco purchase, use, and/or possession (PUP) by individuals under the age of 21. However, the evidence of the effectiveness of PUP provisions in reducing youth tobacco use is limited, particularly among those at higher risk for smoking, and the laws may have adverse consequences for youth already addicted.[8] Learn more about PUP laws.

The Evidence for Raising the Age to 21

In 2015, 4.7 million middle and high school students used some form of tobacco.[9According to the 2012 Surgeon General's report youth initiation is a major factor in the tobacco epidemic. [10]  A model developed by researchers at UC Irvine showed that smoking prevalence for 15-17 year olds would drop from 22% to 9% in only seven years if the age of purchase were increased to 21 across the U.S. [11] Models in a recent report by the Institute of Medicine suggest that smoking prevalence overall will drop significantly between 2015 and 2100 due to previously instituted tobacco control policies even with the MLSA at the status quo. However, they project that  smoking prevalence would drop by an additional 12% if the MLSA were raised to 21, compared to only an additional 3% if the MLSA were raised to 19.
Tobacco use is costly to society.  Every year, smoking costs the US over $289 billion in health care costs and lost productivity. If the MLSA were raised to 21, simulations project a net cumulative savings of $212 billion dollars through decreased projected prevalence of tobacco use and the subsequent savings in medical costs.[12Population health gains in terms of both length of life and health-related quality of life are likely to be 7 times greater when youth smoking initiation is prevented, rather than encouraging cessation among adults once they are already smokers.[13] According to the Institute of Medicine, if the MLSA were raised to 21 now, it could prevent 223,000 premature deaths, 50,000 deaths from lung cancer, and 4.2 million years of life lost.
A higher MLSA limits social channels through which youth can get enough cigarettes to develop a regular smoking habit. Youth frequently rely on getting cigarettes from the 18-20 year olds in their social circles.[14Raising the MLSA reduces access to legal buyers in their daily routine (especially at school) and limits successful store purchases.[10]
Recruiting young adults as replacement smokers has long been a tobacco industry strategy to sustain their business. Smokers who start at an early age are more likely to become regular and heavy smokers.[13] But as one RJR Reynolds researcher said in 1982, RJRquote_0 Motivated by this knowledge, the tobacco industry markets directly to 18-21 year olds through promotions and marketing at bars, clubs, and parties in college towns as well as tobacco corporation-sponsored music and sporting events, capitalizing on this age group's transition to adulthood as an opportunity to hook them on nicotine.[1015
While enforcement will require regular compliance checks, raising the MLSA will create less ambiguity for sales clerks.[10With the legal age currently set at 18, teens who look older than 16 have a higher success rate in purchasing tobacco products.[11] However, if the legal age is raised to 21, the same legal age to purchase alcohol, ID checks could become streamlined and less burdensome for retailers.[6 Smokes 103  

Best Practices for Getting Started

More Resources:

Addressing the Arguments Against Tobacco 21

Concern
Reality

Impact on Retail: Raising the minimum legal sales age to 21 would hurt business for tobacco retailers
Smokers in the 18-20-year-old age range make up 3.06% of the total adult smoking population, and they account for 2.12% of total cigarette consumption in the US. [3If the national MLSA was raised to 21, tobacco retailers would only lose up to 2% of total cigarette sales. [3Tobacco retailers could then adjust to the changing market conditions and the longer-term effects on smoking prevalence just as alcohol retailers did when the drinking age was raised to 21.[3] Not all 18-20 year olds will quit immediately, meaning the effect will be gradual, and the money that individuals previously spent on tobacco will likely be spent on other products. In the four years following Needham, Massachusetts’s law changing to legal age to 21, high school smoking rates dropped 47% in 4 years following the change, but no tobacco retailers have gone out of business as a result. [3]   

Community and state bans may have limited impact because individuals can still cross the state line or go to another town to purchase tobacco.
During the following four years after Needham, MA raised the minimum legal sale age to 21, smoking rates decreased over twice as much as rates in surrounding towns. Read the Tobacco 21 Massachusetts for more information.

Since 18 is the legal age for voting and military service, 18 should be the age for purchasing tobacco, too.
Brain development is not complete until the 20’s, and nicotine may chemically alter a teen’s developing brain, making them more susceptible to nicotine addiction later. At this vulnerable age, teenagers may also incur more lasting damage from tobacco smoke. In addition, some military leaders are supportive of Tobacco 21 policies due to tobacco's impact on military readiness. For more information see "Raising the Tobacco Sale Age to 21 and the Military" from the Campaign for Tobacco Free Kids.
Find more responses in support of Tobacco 21 here.