Psychologists, parents, the government, private donors -- just about everyone believes that a child's earliest years are the most important for their development.
Rather than address that on theoretical grounds, let's cut right to the reality checks. I'm sticking with social-emotional development since that takes a back seat to cognitive development in the broader culture. Parents are if anything eager to teach their kids how to read, do arithmetic, and so on. But they do everything they can to shelter them from the extensive peer contact that drives social development away from childish egocentrism and toward more mature give-and-take relationships.
There is far more variation across the historical record than can be reproduced in the typical lab experiment, unless it's got funding out the wazoo. And because so much of history is cyclical, each phase of the cycle is shot at replicating the original result. I'm going with very recent history because that's all still in people's minds = less work.
I can get along socially with people from my parents' cohort (1954 and '55), all the way up through my own (1980), my brothers' ('82), and even into those who were in high school or college with me when I was a senior ('83 and '84). Reaching back into the earlier Boomers, things get chancier, and I sense a sharper divide between them and the youngest Silents (mid-'40s births). And this has been true all my life, whether folks in my parents' cohort were 30, 40, or 50, whether the Silents were 40, 50, or 60, and whether my peers were 10, 20, or 30. There are very strong cohort or generational affinities that last over time.
What is shared across all those cohorts who I can more or less relate to is growing up in a rising-crime period, the main determinant of the zeitgeist. Now here's where we can test ideas about how crucial the first 3, 4, or 5 years are to a kid's social development.
If children imprint so heavily on their environment up through age 4 or 5, people my parents' age should show a pronounced stamp of the 1950s culture, but they do not. It's not simply that they show a little influence of the '50s, though more of the '60s, which they spent more time in. They just don't show that '50s impression at all.
Earlier Boomers (late '40s / early '50s births) kind of do, so kids must start imprinting on their social environment for real around the time they head of for elementary school. We can't tell whether that's some universal time schedule programmed into human nature, or an artifact of our society sending them into heavy peer contact at that age. But in our society anyway, that's when kids start to really absorb cues about what social world they're going to have to fit themselves into.
At the other end, there are the Millennials born in the second half of the '80s who come from outer space, social-wise. The first cohort that feels totally Millennial is the '87 births, although '85 and '86 are still out there. They spent their first four years in the good old days, granted the twilight of it, but still. If they imprinted early, they'd look like those who were children in the earlier portion of the '80s.
Yet they're more awkward, closed-off, glib, dismissive, and repressive in their social-emotional ways. Way more than people born in '82, who I know a lot of from my brothers and their friends, as well as younger people I knew / hung out with in high school, college, and today. The '82 cohort is 5 years apart from the '87 Millennial cohort, so go forward another 5 years to see if it's merely an age gap problem.
Nope, people born in '87 and '92 are remarkably similar to each other, probably some subtle differences, but clearly Millennial (someday we'll be saying "early Millennials"). The '92 Millennials grew up entirely within falling-crime times, whereas the '87 ones got to live through toddlerhood during rising-crime times, yet their outcomes are not worth splitting hairs over. Likewise, in the previous great generational divide, there's a much greater difference between '43 and '48 births (Silent vs. Boomer) than '48 to '53 births (both early Boomers).
If social development doesn't seem to really get going until age 5 or 6, when does it end? I'm tempted to say it's like language, an obvious rather than a lazy analogy in this case. So somewhere in the early half of adolescence, probably no later than 16 or 17.
If that's right, then we should see a perceptible though not yawning chasm between those who were born from the mid-'50s through the first half of the '70s, vs. those born in the second half of the '70s and first half of the '80s. The first group spent their entire formative years, 5 to 17, in rising-crime times. They obviously have other differences, but not too much in terms of fundamental social skills and behaviors.
Once you get to those born in the second half of the '70s, they're mostly inoculated from the contagion of '92 and after, but not entirely. They spent some sensitive middle adolescent years in that period. Most of their growing-up period told them to develop in one way, and suddenly they got the message to develop the opposite way.
And that semi-awkwardness should show a gradual increase up through the '84 births, who got up through their "middle years" of childhood in the good old days, but spent even more of their crucial years in the atomized era. That sounds about right. The later '70s births do show a stronger imprint of '90s sarcasm, irony, etc., than the early '70s births do. Much stronger. Not that they pioneered it, but that they've internalized it more. Ditto the early '80s births.
With Millennials, it's not semi or quasi-awkwardness, it's full blown. Unlike the gradual cline that we see at the upper end of the window, the lower end shows a sharper divide because children are apparently not very tuned-in socially before age 5. They're still interacting only with close kin, perhaps still being breast-fed. So whether you got in 1, 2, 3, or 4 years before things started going downhill, you all end up the same because all were equally shielded from the larger social stirrings.
Most of this seems on the right track, though certainly I could be off by a couple years at the lower or upper end. The main point is to show how much you can figure out just from review of recent history, though of course it's better to go back to another generational cycle and try repeating the results. I do think you see the same big split between early '20s and late '20s births that you see with early '80s vs. late '80s, only then it was Greatest Gen vs. Silent Gen. But that would be for another post.