After years of playing, the hammers and other action parts will be quite worn. Years of seasonal changes cause bass strings to sound dull and treble tone to lose clarity. Eventually, adjustment alone will not correct these problems, and some parts will need replacing to restore the original tone and touch. Over fifty years A few geographic areas with mild climates have older pianos still in good condition. The needs of the pianist are the real variable in judging a piano's useful life.
Good performance requires a piano in good condition.
Even economy grade instruments can often be dramatically improved by judicious reconditioning. Your piano technician can help you make this decision. Eventually, it becomes less and less practical to continue maintaining a very old piano. The undeniable end of a piano's life comes when the repair cost exceeds the value of the repaired instrument. Rare and historically important instruments may never reach this point unless totally damaged in a fire or other disaster.