When we read through the NIV of Luke 1:13 - 14, it is clearly stated that Zechariah was told by the Angel of Lord that his 'prayer' had been heard from God. Was it that same time he was praying or was it several other times he came on duty? If be so for the first time of his prayer then perhaps the old man never needed a child so soon. But Luke stated a saliant point that they were both advanced in age and his wife was barren, claiming that she had been having sex with her husband but no production of a kid since barrenness is seen.
Luke 1:13, 14 - But the angel said to him: “Do not be afraid, Zechariah; your prayer has been heard. Your wife Elizabeth will bear you a son, and you are to call him John. He will be a joy and delight to you, and many will rejoice because of his birth