。
It's hard for old people to change their minds.
Literal
Old-people [subject-が] thoughts [object-を] change [nominalizer-の] [topic-は] difficult.
A simple judgment about resistance to change. 考えを変える ('to change one's mind') is the everyday collocation. ~のは難しい nominalizes the action with の and pronounces it hard, the basic frame for stating that something is difficult to do.