I was once "fired" because I was the "old hand" in a department that had a sudden influx of developers over ten years junior to me. Yes, I sued and won based on age discrimination.
From my standpoint, managers hire younger workers because they'll work longer hours for less pay, and are less likely to have the "encumbrace" of families to keep them from working OT, or that call them away because someone's home sick, or has to be run to an appointment.
Also, the boss usually prefers people his own age who'll go drinking at Bennigans every night with him.