Something that I read today-- regarding teenage pregnancy and how back in the day teens having babies was normal (as if we can compare our society and teens' maturity level today with those of the Middle Ages)-- prompted me to address this.
We have a misconception-- due largely to a misunderstanding of what "average life expectancy" means-- that people only lived into their 40's, and therefore married very young.
In 1910, the average life expectancy for people in the United States was 50 for men and 54 for women-- and many people who come across this statistic would mistakenly conclude that this means that most Americans in 1910 died in their early to mid 50's.
The key to understanding average life expectancy is that it is an average-- that is, a column of numbers totaled up and divided by the number of numbers added. It's not a mode (the number appearing most often in a set of figures) or a median (a number that represents the "middle"of a group of figures, so that half of the figures are lower than the median and half are higher). An average can be skewed by a few very low or very high figures.
And this is why the average life expectancy of human beings was so low: high infant and child mortality. While many people in earlier ages lived into their 70's, 80's, and even 90's, about 1 in 4 children would die before they reached age five.
Imagine that you have a class of 20 students taking a test. 15 kids score 100, and the remaining 5 score 30. The class average would be 82.5-- but it would be inaccurate to conclude that most students in the class scored C's. The reality is that most students aced the test, and a few failed badly.
Likewise, when you throw numbers like 2, 1, and 3 in with numbers like 77, 82, and 74, that brings the average down.
Infant mortality has decreased dramatically over the past 100 years, and our average life expectancy has risen in response.
Marriage age is another issue that is related to this... the logic goes that, since people didn't live as long, they must have gotten married much earlier. What I have found in the years I've been doing genealogical research is that, at least in America and western Europe, the average age of marriage has remained pretty constant for the past 1,000 years or so: 18-22 for women, and 22-26 for men. Of course, there are women who married at 15 or 16, but that was not too common. It's only been the past three decades or so that the average marriage age has risen-- I think it's about 26-29 for women and 31-35 for men. People are holding off on marriage and having children, largely due to societal and economic issues.