teach an old dog new tricks, one can't/it's hard to

you can't teach an old dog new tricks

proverb You cannot teach some new skill or behavior to someone who is set in their ways. Good luck getting Grandpa to start going to yoga with you. You can't teach an old dog new tricks.
See also: dog, new, old, teach, trick
Farlex Dictionary of Idioms.

you can't teach an old dog new tricks

If you say you can't teach an old dog new tricks, you mean that it is often difficult to get people to try new ways of doing things, especially if they have been doing something in a particular way for a long time. The low levels of participation among older people are affected by the widespread belief that you can't teach an old dog new tricks. Note: This expression is often varied. For example, if you say you can teach an old dog new tricks or an old dog can learn new tricks, you mean that it is possible to get people to try new ways of doing something. Our work shows that you can teach an old dog new tricks. An old dog can learn new tricks if he has both the will and the opportunity.
See also: dog, new, old, teach, trick
Collins COBUILD Idioms Dictionary, 3rd ed.

you can't teach an old dog new tricks

you cannot make people change their ways. proverb
See also: dog, new, old, teach, trick
Farlex Partner Idioms Dictionary

teach an old dog new tricks, one can't/it's hard to

The elderly cannot or will not change their ways. This saying dates from the sixteenth century, at first appearing with a literal meaning in a book of Husbandry (1523) and then in John Heywood’s 1546 proverb collection. It is still current.
See also: dog, hard, new, old, one, teach
The Dictionary of Clichés by Christine Ammer

You can't teach an old dog new tricks

Getting people to change their habits or acquire new skills is impossible. Puppies are teachable, but older dogs are less apt to be able to be trained, or so popular wisdom had it. By the same token, an octogenarian who has read the morning newspaper for decades is unlikely to be willing, much less eager, to switch to the online edition.
See also: dog, new, old, teach, trick
Endangered Phrases by Steven D. Price
See also:
  • a good example is the best sermon
  • teach a man to fish
  • it takes a village
  • village
  • a burnt child dreads the fire
  • burnt
  • bad workers always blame their tools
  • better bend than break
  • the best-laid plans
  • the best-laid plans go astray