I dont think your life expectancy really makes an impact on when people want to have kids. How often do you think about how long you have to live? More often peoples life decisions come when they are forced into a situation to consider it, not some arbitrate measure that people often don't even know about. To many cultures you become an adult (and thus things like family) when you become a contributing member of society, ever expanding education pushing the age in which this happens further and further back. When education stopped at around the start of the teens and you where expected to go to work then, you where forced to consider your life as an adult and therefore things like starting a family. When high school education came to be common this was pushed to the late teens. With tertiary eduction being common it gets pushed into the twenties. Add this to the double income situation we have and families are regularly pushed right back to the 30s. Whist life expectancy was down the 40s, it was not because a 40year old was considered an "old" person, it was that many people died young from disease and accidents which cased the average to be so low.