I believe cadence CAN go down with age -- say, after 75-80 years of age; up until then, either a medical condition or just plain lesser fitness would be the only reasons. TOP SPEED, of course, is a different story!

In the 70's as a teen, I know I was a masher; my speedometer back then was mech, so there was no cadence measurement, and I didn't even know about cadence! But I could regularly, and repeatedly, break 30mph in traffic. (10-spd Schwinn)

Today, on my MTB, I haven't touched 30mph in about four years. I'm normally in the mid-to-high 80's for cadence, and fatigue will make me downshift -- creating an 'artifical' higher cadence -- before I will slow my pedal stroke.

The funny thing is, over any distance under 10-12 miles, I'm faster now than I was at 16! HOO-RAH for maintaining speed!