If a person is qualified to work at the establishment, then they can sell tobacco – if they are indeed an employee of that establishment. They have to be at least 16 to work in a place that sells a lot of alcoholic beverages. Please see statutes below. Also, please keep in mind that the law is different for tobacco than it is for alcohol. A person has to be 18 to sell/serve alcohol.
MO Revised Statutes Chapter 294 - Child Labor – Section 294.040
A child under sixteen shall not be employed or permitted to work by any person, firm, or corporation in connection with:
Any place or establishment in which intoxicating alcoholic liquors or beverages are manufactured, bottled, stored, or sold for consumption on or off the premises, except in establishments where at least fifty percent of the gross sales consist of goods, merchandise, or commodities other than alcoholic beverages.