Meaning of lines 46-50 "When old age shall this generation waste,Than ours, a friend to man, to whom thou say'st.'Beauty is truth, truth beauty'....
Generation after shall come to live on this beautiful world. Each generation will have its unique shortcomings and problems. Ageing is a natural phenomenon and all human beings will perish one day( unlike the characters immortalised on the urn, like the singers and lovers who sing and run after maidens respectively). However, this "unravished bride of quietness" and the foster chilf of silence and time will always act as a friend of humanity.
This urn which is a living example of immortality of art will prove to the world that Beauty and Truth are two sides of teh same coin. What is beautuful has to be true and what is true has to be beautiful. This is all that the human beings need to know and this alone will serve them in life.
One must also remember that indian mythology also stresses on the same aspect through the statement, SATYAM, SHIVAM, SUNDARAM. i.e, what is true is shiva, what is shiva is beautiful.