Speaking from a purely composites point of view,...I'd be surprised if they don't get softer with time. Every time you bend the rod on a snag or fighting a fish,...and to a lesser extent, even when casting,..you're doing a tiny bit of damage to the graphite composite. This will build up over time, to the point where the rod will have lower flex modulus (stiffness) and you will notice it as "softer action". How fast this happens is gonna depend on the rod and the user.
BTW, I'd expect this to actually happen more with a high-end rod,..because they are lighter (ie less material) than the cheaper ones.
