It speaks to a common conception - both in the valley and in tech - that this is a business for the young.
Most programming work doesn't require a genius level iq. It requires being able to build simple things very, very quickly. As a result, the experience level at a company matches the structure pyramid. The smart, older guys at the top make all the tough decisions and the fresh, younger guys at the bottom code as fast as they can. As those at the bottom age, they find themselves competing with other programmers their age for far fewer jobs toward the top of the pyramid.
Most programming work doesn't require a genius level iq. It requires being able to build simple things very, very quickly. As a result, the experience level at a company matches the structure pyramid. The smart, older guys at the top make all the tough decisions and the fresh, younger guys at the bottom code as fast as they can. As those at the bottom age, they find themselves competing with other programmers their age for far fewer jobs toward the top of the pyramid.