Definition
This idiom is used to express that it is very difficult to teach someone new skills or change their established habits or character, especially when they are older.
I tried to teach my father how to use a smartphone, but he couldn't get the hang of it. You can't teach an old dog new tricks, I guess.
He's always late, and even though I keep reminding him, he can't change this bad habit. You can't teach an old dog new tricks, you know.
My mother always cooks with traditional methods, even when I introduce new recipes, she insists on using the old ways; you can't teach an old dog new tricks, after all.