Its because people get other priorities with age. They dont care about learning new things as much, as a general rule. They have their paycheck and they value time with family or friends. The job is just a way to get money, not a choice or a passion.

This is in contrast with young people, who often wants to show how good they are, make a career, get recognized. Believe it or not, those older people wanted that too but not anymore. They did it, but other things became more important than work.