https://en.wikipedia.org/wiki/Electromigration

Temperature is a big factor, as well as current density.

But there's also the # and magnitude of thermal cycles (which translate into mechanical stress, leading to metal-fatigue like effects on contact points etc), attack from chemicals in the air, cosmic radiation, ESD damage & more. Some may matter, some not.

That's why "new" > "used" in case of electronics. Especially since you don't know the (ab)use history of used parts.