PDF (1989 edition)
A book about the history of age norms in America. Not only is our modern notion of age a social construct, it's a pretty recent one, largely developed in the late 19th and early 20th centuries. The book doesn't go into great detail, but it's a good overview.
A lot can be traced back to the formalization of education. Schools were divided into grades, and from that followed ideas about what was normal for children to be learning and doing at a given age — all manner of bullshit treated as scientific simply because it was superficially formal and rational. Capitalism's also a factor, of course, both in efforts to get the most fit workers into the workforce and the commercialization of birthdays and age-related merchandise. And a lot of it seems to be everyone just going along with it.