Старого учить, что мертвого лечить

You cannot make old people change their way of life, ideas, methods of work, etc. See Молод - перебесится, а стар - не переменится (M), Старого пономаря не перепономаришь (C), Старого пса к цепи не приучишь (C)

Cf: It is hard to teach an old dog tricks (Br.). An old dog cannot alter his way of barking (Br.). An old dog will learn no new tricks (Am., Br.). An old dog will learn no tricks (Am.). You can't teach an old dog new tricks (Am., Br.). You can't teach an old horse new tricks (Am.). You might as well physic the dead as give advice to an old man (Br.)