Adam Clarke on Psalms 71:18

Old and grey-headed - In the ninth verse he mentioned the circumstance of old age; here he repeats it, with the addition of hoary-headedness, which, humanly speaking, was calculated to make a deeper impression in his favor. Though all these things are well known to God, and he needs not our information, yet he is pleased to say, "Come now, and let us reason together." And when his children plead and reason with him, they are acting precisely as he has commanded.