Basically, any Airstream product built before 1975 will need its axles replaqced. You can tell if it needs them now by looking at the angle of the torsion arms, and by bouncing around inside the trailer. If the hub part of the torsion arm is above the cener line of the axle, they need to be replaced, and if when you jump up and down in the trailer it doesn't really bounce, or "give", you need new axles.
For further info on axles, you can go to www.inlandrv.com
, and check the axle info provided there.
Also, the absolute worst thing for torsion axles is to sit for 20 years without moving, they will "sieze".