It wasn't long ago that age was equated with knowledge. The apprentice learned from the master and the disciple from the guru. Older workers earned higher salaries because of their experience.
Even today, we feel more comfortable with doctors, airline pilots, and Presidents who have grey hair. Yet older workers in many industries can't even get interviews. When they do get job offers, these are often at lower salaries. In Silicon Valley, investors openly talk about their bias towards younger entrepreneurs -- some argue that Internet entrepreneurs peak at the age of 25.