Final answer:
A person must be at least 18 years old to sign a life insurance application. This aligns with the legal age of adulthood in the U.S., at which individuals become legally responsible for themselves and can enter into contracts.
Step-by-step explanation:
The minimum age at which a person can sign a life insurance application is generally 18 years. Adulthood and the age at which one can take on legal and financial responsibilities largely begins at 18 in the U.S. This is the age at which a person becomes legally responsible for themselves and can engage in various contracts, including signing for life insurance. While adulthood and aging can vary, and one can be physically mature by ages 16 or 17, it is the legal definition of adulthood that dictates dealings in legal and financial matters, such as life insurance policies.