I think the ageist position is that experience and wisdom aren't enough to counteract the brain's decline as one ages. So really this is an argument about two things: 1. The rate of cognitive decline and 2. the value of experience. The former varies with each individual and can be mitigated with proper nutrition and exercise. The latter varies depending on the task.

Oh, and in case anyone disputes that cognitive abilities decline with age: http://en.wikipedia.org/wiki/Memory_and_aging#Memory_decline...