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) related to cognitive development. It is used to assess the level of cognitive skills a person has, which corresponds to the age at which typical individuals acquire those skills. The first option is about physical fitness/health (not mental), and the second about life expectancy (unrelated to mental age's purpose). The third option matches as mental age helps approximate when certain cognitive/skill - related milestones (skills) should be acquired.

Answer:

to approximate the age at which certain skills should be acquired