When a person is brain dead, you can legally turn off life support. This is because there has never been a case of a brain dead person resuming normal life. There is literally no chance of that person ever being viable again.
With regard to birth, the date of viability has rolled earlier as our medical abilities have improved. Babies are now born at dates that were unthinkable a decade ago. Another aspect of infant viability is different as well. One can say with near certitude that the fetus at six weeks WILL BE viable in a matter of weeks.