You cannot teach an old dog new tricks.
Established habits resist new instruction.
What They Mean
Do not schedule the training. After a certain age the routine is load-bearing, and new procedures will be politely ignored.
Are They Right?
About dogs, debatably. About people, the saying confuses cannot with will not, which is a different diagnosis requiring a different form.
What They Left Out
The old dog already knows tricks the new dog has not heard of.
They Also Say
Read the ruling: You Are Never Too Old to Learn vs. You Cannot Teach an Old Dog New Tricks