This were to be new made when thou art old, And see thy blood warm when thou feel'st it cold. (lines 13-14)
Then, all of a sudden, after this grim stuff, the speaker switches it up on us! Life is supposed to go from good to bad, from strong to weak, etc. Now, though, things have changed. Thanks to the magic of having kids, old things can become young, cold blood can warm up again. What's happening here is that time is being reversed, the flow that has only gone in one direction (from young to old) is being turned back.