Mercedes says the older pre 1992 cars need the fluid flushed every year. I do all of mine ever 2, or as needed with component replacement. From my owner's manual:
When I did the fluid on my sister's grand prix (which was 6 years old at the time? the fluid was original.) the fluid that came out was black, and the brakes felt much better afterwords. And GM says it's a lifetime fluid
-J