Must confess I never changed the brake fluid in my 1988 Volvo 760 Estate until I had to replace a rear caliper, I will not tell you why I had to replace it....????
Auto trans fluid is in a virtually sealed environment, the fluid relies on temperature stability to maintain its structure, if my memory serves me well, its 175*F. If optimal temperature is maintained it can go on forever, 100,000 miles is no problem for auto fluid if temp has been in its operating window in those old boxes. ????