MB pads and rotors do wear out quicker than on most makes, but 21,000 miles is quite quick, even for a Mercedes.
I wouldn't get too excited yet, not until a second opinion is found.
On my MB cars, I've found the fronts and rears wear out at nearly the same time, mostly around the 60-70,000 kilometer (35-40K-miles) mark.
Also, the light comes on for the front pads only, and at about 10% remaining. There's nothing wrong with waiting until the light comes on, as there's still time before metal-metal happens.
1998 C230 "Black Betty"