A member may retire and commence to receive retirement income payments on the first day of the calendar month following the board's receipt of the member's voluntary written application for retirement or the required beginning date, if earlier. Before receiving retirement income payments, the member shall have ceased covered employment and reached early or normal retirement age. The retirement income payments shall be in an amount as provided under section eleven of this article: Provided, That retirement income payments under this plan shall be subject to the provisions of this article. Upon receipt of a request for estimation of benefits, the board shall promptly provide the member with an explanation of his or her optional forms of retirement benefits and the estimated gross monthly annuity. Upon receipt of properly executed retirement application forms from the member, the board shall process the member's request and commence payments as soon as administratively feasible.