Saying you can or can't teach something to someone once they've gotten past the
time when they learned their ideals/ways. Saying you can or can't change someones mind about their
long term opinions or beliefs. can be referring to a
human as well.
A: my dog won't listen when I tell him to sit!
B:
yeah, you can't teach an old dog new tricks.
A: but can't you just hear me out on this, B?
B: no! I made up my mind a long
time ago.
A: *
sigh* you can't teach an old dog new tricks.
A: I'm 70 years old but I just learned rock climbing!
B: you can teach an old dog new tricks!