Answer :
The author's conclusion that it's hard to teach an old man new tricks simply means that it's difficult to change the behavior of a person.
It should be noted that it's difficult to change the way that a person does something when the person has been used to that thing and doing it for a long period of time.
Therefore, when an individual is old and doing things in a certain way, such a person finds it difficult to learn something new. Therefore, it's hard to teach an old man new tricks.
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