You can’t teach an old dog new tricks
if you have been doing something for a long time it’s hard to change.
e.g. I feel really sorry for our office manager at work. After 30 years’ experience and with one year to go before she retires the company has adopted completely new systems of working. Everyone knows that you can’t teach an old dog new tricks.
The idiom originated by Heywood in 1546 and is considered as one of the oldest idioms of old English language.