The rubber tends to harden due to "gassing out" (the petro chemicals are slightly volatile - hence the strong smell of a new tire) over time, ozone, engine bay temperature, and sunlight UV. This will affect how much and how easily the hub can shift inside the rim, which is how the crank harmonic is suppressed: converted to friction heat in the rubber. It will still do something, but not at the designed engine speed, and perhaps to a lesser extent. A new one should be good for at least a decade (just my guess).
