I think that by the middle age people are more settled; they seek more security and more stability, and striving for occupational success is sometimes combined with a risk that people at this age no longer accept.
Additionally, many people have children and grandchildren that they would like to spend time with, and they also value family and free time more than work.
Furthermore, middle age is when people's health starts to deteriorate, so they might not be physically able to work as much.
