Adam Clarke on Psalms 71:9

Cast me not off in the time of old age - The original might be translated and paraphrased thus: "Thou wilt not cast me off till the time of old age; and according to the failure of my flesh, thou wilt not forsake me." My expectation of rest and happiness will not be deferred till the time that I shall be an aged man, Thou wilt not withdraw thy presence from me as my flesh decays, and as my natural strength abates; but, on the contrary, as my outward man decays, my inward man shall be renewed day by day. It was in David's old age that the rebellion of Absalom took place.