Section 120A. No insurer authorized to issue policies upon the lives of persons in the commonwealth may refuse, for the sole reason of mental retardation, to issue a policy of individual life insurance on the life of any person residing in the commonwealth who has reached the age of three years, if there is no other insurance in force or pending consideration on the life of such person. Insurer as used in this section shall include every life insurance company as defined in section one hundred and eighteen and every fraternal benefit society as defined in section one of chapter one hundred and seventy-six.