Strictly speaking, most doctors define the age of viability as being about 24 weeks of gestation. In many hospitals, 24 weeks is the cutoff point for when doctors will use intensive medical intervention to attempt to save the life of a baby born prematurely. A baby born at 24 weeks would generally require a lot of intervention, potentially including mechanical ventilation and other invasive treatments followed by a lengthy stay in a neonatal intensive care unit (NICU).
In the hands of experienced specialists, though, babies born slightly earlier may have a chance at survival. Babies born at 23 weeks may survive with these specialists in a state-of-the-art NICU, but the odds of survival are much lower. The earliest baby to have ever survived premature birth was born at 21 weeks and 6 days, and this was reported in the news as having been a "miracle."