describe why job satisfaction tends to increase with age

Job satisfaction is determined by the the variance between job expectation and the actual result. If your actual result exceed your expectation, thats what we call job satisfaction. E.g YOu satisfy with your job because you get a good payroll that exceed your expectation.

Answer for your question,there are a few phenomenon on aging staff:
1. When they get older, they are looking for stability instead of having new challenges and expectations.
2. When age increase, which mean you have been promoted to a certain level in a company
3. Work life balance: Instead of pursueing goals at workplace, family has become another important factor.

Those phenomenon will affect your expectation towards a job and you intend to accept the current situation in workplace in order to look for stability, instead of finding other way to solve it. So the job satisfaction tends to increase with age, because their tendecny to remain status quo and other factor like family.