Should a serpentine belt be replaced after age of belt or mileage driven?

I have a 2002 Ford V-10 Triton w/33,000mi.

A serpentine belt should be replaced when it shows signs of incipient failure. They can be visually inspected.

The books will give you a rough idea when, but like Helena says, do a visual. Grab a section of the belt and try to flip it over to see the under side. Look for hairline cracks.