Oh Grow Up!
Stereotypes
We have a number of stereotypes about people and age, without even realizing we have them. Here are a few.
Childhood didn't exist
Children were "little adults." They were dressed in grownup clothes and at a very young age were put to adult tasks.
Adults didn't care for children as much as we moderns do because so many children died so young. It was a defense mechanism; plus, childhood itself was barely recognized as a stage of life.
Adolescence is a modern invention, needed for a trained workforce. Stay in school, modern society says, in order to get a good education and a good job. But in reality society cannot absorb the teenage workforce and so tries to keep them out of the primary job markets. By inventing the idea of a middle ground between childhood and adulthood, we keep teenagers out of the job market. It's also another manifestation of the modern romanticization of childhood.
Childhood itself didn't exist in the Middle Ages. This goes back to the "little adults" point above, but it means that even the childhood years were spent in teaching young people adult tasks, or grooming them for such tasks. Childhood as an idyllic period of innocence was invented by the Romantics in the 19th century.
People got married really young, like at twelve or ten or even younger. This, too, contributed to the near non-existence of childhood.
The Invention
This perception of the past was constructed by some particular writers, including Edward Shorter, Peter Laslett but especially Philippe Ariès. His Centuries of Childhood, written in 1962, was a seminal work. Here's a quote from Chapter 2, tellingly entitled Discovery of Childhood "...there was no place for childhood in medieval society." There's a whole section on how people didn't care about little ones because the little ones died a lot.
This picture of the past is almost completely wrong.