This is probably addressed to an actor who wants to audition for the role of a character who is much younger than he is. It means 'You are far too old to act the role of this character'.

Shakespeare's Hamlet is often called "the melancholy Dane." I'm sure this is what "the Dane" refers to. "Dane" is the word for "people from Denmark." The full title of the play is "The Tragedy of Hamlet, Prince of Denmark." The fictional Hamlet's age is uncertain, see https://en.wikipedia.org/wiki/Prince_Hamlet#Hamlet.27s_age but either he is 30 or he is much younger, perhaps even 16. An actor can act younger than he really is, but there are limits. The passage is saying that it takes years to become a good Shakespearian actor. By time you are good enough to play Hamlet, you are too old to play the part of a young Prince convincingly.