As the S5 is not a featherweight 30k and 6 years does not sound that strange to me for replacing the front rotors. It's a bit early maybe, I would assume with a normal driving style 60k should also be possible.
The back rotors should outlast the fronts as they don't get the same wear. I've replaced front and back on my RS5 recently (180.000 km) and I don't think the rear rotors were ever replaced before.
I don't agree with the statement that the rotors are "thin" or in general should be replaced together with the pads. I've never replaced rotors and pads in the same interval .Usually I can wear through 2 or 3 pads before I replace my rotors. The only time I had rotors wear as quickly as the pads was with aggressive aftermarket pads (Carbon Loraine RC5+). The rotors on the (R)S5 do have a bad reputation, but that has more to do with the size of the disc and the sensitivity to fitting errors. A slight misalignment will lead to uneven build up of friction material and shudder in the brake pedal. The bigger the rotors, the more noticeable the effect will be.
