Throughout the United States, to my knowledge, there are minimal ages for buying tobacco but not for actually using it.
Also, see:
http://www.gamefaqs.com/poll/index.html?poll=3864
which Utar referred me to in this rather similar topic:
http://arfer.net/pms/topics/55832813
It's strange that this poll has no option "I know people who do, but they ain't no friends of mine."
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"You blockhead!" -Cheez