Tobacco and alcohol origin cancers take years of exposure to cause the problem when you look at long term demographics. The average oral cancer from tobacco, is in a person in their late 5th through 7th decade of life, who have been tobacco users, of many "pack days" for a protracted period of time. While there are no absolutes, and while documentation of young people getting cancers who also smoke is there, it is not a large number. But it is certainly not zero either.