You can't teach an old dog new tricks.
As we age, it's harder to learn new things. It is impossible to change someone's ways or habits, especially if he is old and resists change.The expression is an excuse for those of a certain age who have tried a little and failed, and who can't be bothered to try at all, and is often used when people have just given up or are about to do so.
- Nobody said it was easy to teach an old dog new tricks, ... (you may use this variation to encourage someone to continue their efforts)
- Who says you can't teach an old salad* new tricks? (=Who says it's impossible to change or improve it?)
- You can't teach an old hound* new tricks.
* People occasionally substitute the word with an asterisk with a different one, not necessarily a synonym of "dog", for example, "guard", "politician", "man" etc.
It's hard to make an olde dogge stoop.
It's hard to teach an old dog tricks.
An old dog will learn no tricks.