1. Managers that want to hire people they could be best friends with. Drinking buddies, social media socialites, etc. Don't confuse this with hiring their best friends, they just feel more comfortable around people that they could socialize with.
2. Managers that don't want anyone to undermine their technical authority. And the best way a manager with 5 years of experience can keep his authority is to not hire an engineer with 20 years of professional experience.
3. Managers that think the culture of a company would turn more boring if they hired middle-aged workers. So they must hire young people only. They can usually get away with it if they hire 80% college graduates mostly.
4. There must be something wrong with the person if he didn't get into management by the age of 50.