It's going to depend on the car/truck and year model. Generally, the axles and transmissions should be changed every 30,000 miles. Axles especially if they are they traction lock type as the fluids will contain alcohol (friction modifiers) for the clutch packs. Alcohol will absorb moisture out of the air- it's called hygroscopic, and will sludge. If the unit uses the newer DOT 5 brake fluid, it's a silicone fluid and is also hygroscopic. You can mix DOT3 and 4 but not 5. It should be flushed at every brake pad change. Most of the newer cars come from the factory with coolant that should last 5 years. Make sure to use the correct anti-freeze as there are far too many now. DO NOT buy into the "one is good for every car" as they are a compromise. Use the correct speced fluid. Power steering fluids are really more like a hydraulic pump. They really don't get all that hot but hot enough to cook off any moisture. They aren't contaminated with any clutch parts, etc so they don't have to be changed. But I would suggest at around 80-100,000 miles to suck out as much as possible and refill with the correct fluid. That will keep any varnish solvent.
There are no lifetime fluids to serviceable systems. I've seen several lube for life systems that use an oil filter that's about 1 gallon in size. It contains enough additives to last 100,000 miles and is time released into the oil. The filter itself is multi-staged with normal positive filtration and then it also uses something very similar to by-pass filtration. They've all worked per design but I doubt you're going to see it. The base fluids were a proprietary ester that was similar to a thin plastic slime. The cost was fairly prohibitive. Fluid replacement at the recommended 100,000 mile interval would cause a lot of folks to have to go to the bank to change it.
