There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. According to studies between 2003 and 2005, 20 to 35 percent of babies born at 23 weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive. It is rare for a baby weighing less than 500g (17.6 ounces) to survive. A baby's chances for survival increases 3-4% per day between 23 and 24 weeks of gestation and about 2-3% per day between 24 and 26 weeks of gestation. After 26 weeks the rate of survival increases at a much slower rate because survival is high already. - http://en.wikipedia.org/wiki/Fetal_viability
There is no specific age or time in which a fetus is considered viable. Medically though, a fetus tends to have 50% to 70% chance of survival if it is born at 24 to 25 weeks gestation. If you mean changing its state by growing, then it does but only if there is advance care as soon as possible.