What should be the legal age to purchase cigarettes (tobacco)?

This largely reflects a bigger debate regarding the freedoms allowed to 18-20 year olds. A lot rhetoric is centered around this age group's ability to go to war and vote. I flip-flop a lot on this issue in my own head. But, the fact remains that these citizens can vote and can legally challenge the government's decisions in this context. It is up to these people to defend their own rights as adults.