Don't assume the original failure was old age. The root cause should be diagnosed. In this case, I'd guess the original failure cause was due to some other part of the setup. Replacement of the affected part may not be anywhere near a cure for the cause.
Garage suggesting camshaft is nonsense. Are you sure they said camshaft?
Outer crank pulley (Which I guess is what we're talking about here) drives the auxiliary belt, not the camshaft. If they are really suggesting camshaft, you need a different garage. Unlikely to be a fault with the crankshaft either. The nose where the pulley fits is unlikely to bend, and any issue there would be visible with the engine running, as it would show a wobbly pulley, so quick and easy to diagnose.
The pulley is a two-part item, using a rubber block to transmit the drive from the inner boss on the crank, to the outer grooved pulley for the belt. I'd not expect either metal bit to fail, unless struck by a rock ro similar, so I'm assuming it is the rubber part that is failing. Even cheap ones will last longer than a few months. Failure is due to loads or shocks greater than expected. So you need to look at each part it is driving. (Alternator, power steering, aircon, tensioner pulleys, water pump?, and anything else) One of these is causing the problem.
The alternator probably has a freewheeling pulley. These fail, and there have been several posts on here, particularly Multipla, where a failed pulley has resulted in cracked alternator casings. A siezing alternator, or any other of the driven components can create higher stresses on the drivnig pulley, adding load and strain to the rubber part.
Your garage need now to diagnose the cause of failure, not the result.