In Psalm 71, David is beginning to fade from the scene. His age makes him the target for those who would seek to destroy him, seeing his weaknesses as an opportunity to take advantage of and his faith being in vain. 

But David knows that God is a greater refuge for him and reflects on other times that he was helpless like when he was an infant and baby and God provided for him then before he was even aware so now will God do now. 

David uses prayers that he has prayed before, often in what is done in old age. Psalms 22 and 31 are quoted and alluded to in great detail. God’s faithfulness and promise concerning the promise of his Son, which is why Psalm 72 is from the hand of Solomon his Son. The enemies’ plan to destroy God’s promise of Christ is thwarted and will always be thwarted.