I think it may also be a perceived long-term cost for health care. Despite more experience and maturity, they may be seen as costing too much in the long term. I worked at a non profit in Atlanta who hired a guy in his late forties. There were many comments that he might not fit into the "youthful culture" of the place, since the average age was 30. Employers want to hire young people because they can work them longer hours, with less health care costs to worry about, and a belief that they'll fit into the company culture more. I've also read about an issue of hiring managers who are young not wanting to hire people older than them.