Final answer:
Regarding near vision testing, the text implies that it's more critical to screen as individuals reach an age where they are more susceptible to vision conditions like presbyopia, typically around age 40. A nearsighted person with a clear vision limit of 20 cm would need to stand 10 cm in front of a mirror to see themselves clearly for tasks such as shaving.
Step-by-step explanation:
The question pertains to the requirement for near vision testing for service members, which suggests a focus on optometry and vision health. The original question about the specific age under which service members do not need near vision testing unless they have difficulties has not been directly answered in the provided reference text, thus a direct answer to the specific age cannot be given with the information at hand.
However, in discussing near vision, we can infer that routine vision screening, including near vision, typically becomes more critical as individuals reach an age where presbyopia and other age-related vision conditions become more common, usually noted around the age of 40.
A nearsighted person, as described, has a limit on the distance they can see clearly, which is myopia. Therefore, if a nearsighted person cannot see beyond 20 cm, they would need to stand just half that distance (10 cm) away from a mirror if they wish to see their reflection at the equivalent of 20 cm from their eyes because the mirror's surface essentially doubles the distance they are able to focus on.