Not always, but a lot of times people pick the number that supports their conclusion. For example, worker pay. If you want to make it seem higher, include the executives and do a mean average. Life expectancy works the same way. It's not that adults typically died at age 30, it's the number is a mean and includes child deaths which was a lot at the time. If you have 4 people, 2 are 60 and 2 died at birth, the mean age is 30. (60 + 60 + 0 + 0) / 4 = life expectancy of 30.