I knew a brother who was a substitute CO and at the time he was in his 40s. Pioneer Elder and ex-Bethelite.
He was one of the good elders. I liked him. At the time (I was still indoctrinated) I was looking forward to him making full CO but it never happened. I don't know why he didn't make it, but I got the impression he was relieved when they didn't offer the position to him.