He asked life of thee, and thou gavest it him, even length of days for ever and ever. The "life" intended cannot be ordinary human life, since in David' s case this certainly did not continue "for ever and ever."
We must understand the psalmist to have asked for continuance in his posterity, and this was guaranteed him in the message which God sent him by Nathan (2 Samuel 7:13, 2 Samuel 7:16). In the full sense the promise was, of course, Messianic, being fulfilled only in Christ, the God-Man, who alone of David' s posterity "liveth for ever."