Psalms 103:4,5 - Joseph Benson’s Commentary on the Old and New Testaments

Who redeemeth thy life from destruction Both temporal and eternal; from deadly dangers and miseries. Who crowneth thee with lovingkindness That is, encompasseth and adorneth thee therewith, as with a crown. Who satisfieth thy mouth with good things Satisfieth all thy just desires and necessities. So that thy youth is renewed like the eagle's That is, as some interpret the words, As the eagle appears to renew her youth with her plumage, when she casts off all her old feathers, and gets new ones, whereby she seems to grow young again. But, as this is common to all birds, it is hardly to be supposed that the psalmist would have alluded to it here as if it were peculiar to the eagle. This circumstance, however, is most observable in hawks, vultures, and especially in eagles, which, when they are near a hundred years old, cast their feathers and become bald, like young ones, and then new feathers sprout out. But the psalmist seems chiefly to refer to the long lives of eagles, and their great strength and vigour at a very advanced age. Hence the old age of an eagle is used proverbially for a lively and vigorous old age.