To be set in his ways means the person is stubborn and does not like change.
A person who is set in his ways is used to doing something one way and has done so for years and does not want to try another way.
Even if doing that thing different is actually better the person who's set in his ways will not want to try something different.
The saying of "he's too old and set in his ways" means that the person is stubborn and will not change or go along with something new or some new way of doing something.
When a person is set in his ways it means that the person is stubborn to rethink or question something.
It's not just old people or older people who are set in their ways as I find many people including myself to be sometimes stubborn about doing something another way.
However most older people learn to do something a certain way and like things a certain way and are unwilling to change their ways of doing things or what they are doing.
The person may also be unwilling to change their behavior when they are set in their ways.
Not everyone is set in their ways just because they are older but it's a common thing older people are found to be doing.