The average lifespan of a bat is 20 years. This is a longer lifespan than most rodents of comparable size, which generally survive only one to two years.
Young bats suffer a high mortality rate as a result of falls from their nest, parasites and disease. By three weeks old, bats are big enough to fly, and their life expectancy rates improve. In one documented case, a brown bat lived until the age of 30. To survive the winter, some bat species migrate, some hibernate and some go into torpor, which is a regulated state of hypothermia lasting anywhere from a few hours to a few months.