From the paper:
Before the start of the industrial revolution in the late nineteenth century, people of all ages inhabited the same social world. Children of all ages were educated together. Children and teenagers worked alongside adults on farms and in factories. Several generations often started one household. Neither children nor old people were set apart from the rest of society on the grounds that they were too young or too old to participate. By the middle of the twentieth century, age consciousness had emerged and people in developed cultures had become accustomed to thinking of life as a progression of distinct ages.
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Medical students had established pediatrics, the treatment of children and geriatrics, and the treatment of old people. Birthday celebrations had become a commercial enterprise.