A fireman who continues to be disabled beyond the maximum period of eligibility for ordinary disability benefits as the result of any cause other than the performance of an act or acts of duty, and who withdraws while still so disabled and before age 50, shall receive the annuity that may be provided from the amounts accumulated to his credit from salary deductions and contributions by the city for his retirement annuity. The annuity shall be computed as of the age of the fireman on the date of his withdrawal.
     The annuity to which the wife of any such fireman has a right from the date of his death shall be fixed as of her age on the date of his withdrawal. It shall be an amount provided on a reversionary annuity basis from the entire amount to his credit for widow’s annuity. The maximum age of the wife for annuity purposes shall not be more than 5 years less than the fireman’s age.
     Upon the death of a fireman after he has entered upon annuity, any unmarried child of his under age 18, shall have a right to receive annuity under the conditions and of the amount specified in this Article for a child’s annuity.