The average life of a piano, grand or upright, is ninety years. Beyond that age a piano may be difficult to tune, go out of tune quickly, or possess an action that clatters and rattles when being played. As the wooden parts in the piano swell with age and/or moisture the keys become stiff or sticky. Sometimes keys will stick down and remain depressed. Such a piano has little value. Some people argue that such an instrument is suitable for a beginner. It is not. A novice player will become quickly discouraged playing on an out of tune instrument that constantly rattles and the keys stick. Avoid purchasing very old pianos. They are a false economy and a considerable amount of money will need to be spent on getting them in working order.
Upright pianos, full iron frame, over strung and under damped from sixty to eighty years old can be useful instruments provided they have been looked after, regularly tuned and maintained and kept in a stable dry atmosphere. Three quarter frame pianos, overstrung and under damped will be worth less and are not as stable. Avoid pianos which are over damped and straight strung. Pianos with spring loop actions are worthless, as are half frame and wooden frame pianos.
Grand pianos aged sixty to eighty years, full iron frame, overstrung and with a roller action will be worth looking at depending on make and condition. Obviously a grand with a well known makers name on the front will be worth a little above the average value. Grands around the ten to fifty year old mark will generally be worth what ever value they had when they were first purchased. As with uprights do not buy a wooden frame grand or three quarter frame grand from the early part of the twentieth century.