The choice indicating that heavy makeup, especially eyeliner, can suggest a customer may be underage is rooted in the observation of grooming trends among younger individuals. In many cases, younger people use makeup as a means of expression or to appear older, which could prompt servers to be more vigilant when checking for age identification. This clue can serve as a reminder for servers and establishments to ensure they are compliant with laws regarding the sale of alcohol to minors.
The other options, such as wearing formal attire, having facial hair, or sporting distinguished jewelry, do not provide reliable indicators of age. Formal attire is worn by people of all ages for various occasions. Facial hair can appear in younger individuals as well as older ones, and distinguished jewelry may be a fashion statement that crosses age boundaries, making it an unreliable clue in determining someone's age. Thus, heavy makeup stands out as a more specific sign that could trigger a server to ask for identification.