What this poem is saying is that the speaker does not want her (his?) love to ever grow old.  The speaker is saying that their love should never be touched by the pen of aging but should stay beautiful forever.  (It also says the loved one will live forever in this poem, though.)

So the line you cite, which is the first line, is saying something like "go ahead, time which devours everything.  Go ahead an make the lion's paws blunt."  It goes on to say what else time can do.  But then it says "okay, time, do all that stuff, but leave my love alone."