Question

what is the concept of mental age used for? to approximate the age indicated by physical fitness and overall health to approximate life expectancy to approximate the age at which certain skills should be acquired

Explanation:

Mental age is a concept in psychology (a subfield of Social Science) that relates to the cognitive and skill - related development of an individual. Physical fitness/health - related age approximation is not what mental age is for, and life expectancy also has no direct relation to mental age. Mental age is used to estimate the age - level of cognitive or skill - based development, i.e., the age at which certain skills should be acquired.

Answer:

to approximate the age at which certain skills should be acquired