There is a minimum age for this work, it is set where you work rather than nationally, and — the part that catches people — it is frequently not one number.
Two different questions
The age to hand someone a drink and the age to stand behind the bar and make it are separate questions with separate answers in a lot of places.
The federal research system that tracks this records them as two different things: it keeps a minimum age to serve and a minimum age to bartend, each broken down by beer, wine and spirits, and a separate variable for whether a manager or supervisor has to be on the premises while you do it.
That is a good picture of the shape of the answer. Serving beer to a table at eighteen and mixing spirits behind a bar at eighteen can be two different legal positions in the same building.