As you get older your ears progressively suck, especially at frequencies >10kHz...You're one of the few people I've ever seen mention this. I mentioned it on my website but subsequently had doubts about the magnitude of the effect because the cantilever's rubber mount should have a much greater damping effect, at least according to some calculations that someone else undertook. Still, I think you're right - the back EMF when a cartridge sees a lower resistance seems a plausible explanation for any change in sound.
What's the Geezer effect?
MC or MM cartridges produce an output due to changes in the linked magnetic flux- the result of which is an output defined by Faraday's law of induction e=-dPHI/dt, where PHI is the linked flux, so the movement of the coil in the magnetic field produces a voltage across the coil (e) and the - sign is, essentially, Lenz's Law.
However, if, at the same time, this induced voltage causes a current to flow through the inductor the acceleration of the electrons creates a field in opposition to the movement, which effectively reduces the linked flux.
This "back EMF" exists as a consequence of conservation of momentum applied to electrons- i.e. electrons cannot change velocity instantaneously so an instantaneous change in linked flux must have an equal and opposite flux generated to oppose it- and it gives us v(backemf) =-Ldi/dt.
Thus, when a generator (cartridge) tries to induce an output voltage that causes a current to flow the flow is opposed.
It's easy to simulate and I include it in my cartridge/preamp models. Note that it's a function of di/dt, so lower load Rs and higher frequencies are what is important. In simulation interesting effects can be seen in the cartridge/preamp response to square waves when loading is altered due to the opposing effects of the electrical resonance and the back emf.
