I own a 98 ML320 and the mileage is about 18 in town and 21 on the highway. Brake pads were replaced at 35K miles. If you ask me, replacing pads before 20K miles is excessive. Might want to have the calipers checked...mine were replaced under warranty along with the master cylinder.
Good luck.

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99 E300 133K
98 ML320 99K
87 Saab 900 Turbo Conv 123K