Actually in Maryland you can Bartend at 18. Most places won't hire someone that young but it is legal. Maryland doesn't differentiate between servers and bartenders. 18 for both.
Now you made me look it up. Its a county by county thing. In MoCo there is no split. This is for Chuck Co*:
Age of Persons Serving Alcoholic Beverages
In Charles County, a person eighteen (18) years of age or older may serve alcoholic beverages in restaurants in connection with the serving of a meal. Persons under twenty-one (21) years of age may not act as bartender or barmaid in any solely bar-related capacity in Charles County. (For exact language see Article 2B, Section 12-209.2 Annotated Code of Maryland.)
Employment of Minors by Retail Dealers
In Charles County, a person under 21 years of age may not be employed by any holder of a Class D beer, wine and liquor license in the sale of alcoholic beverages and no person under the age of 18 shall be employed for the sale of alcoholic beverages in any other licensed establishment. (For exact lan-guage see Article 2B, Section 12-302, Annotated Code of Maryland.)
* we are a 'code home rule' county, so every county regulation is a state law. MoCo otoh is a 'charter government' county, so county regulations are passed by the county legislature.