Variability of Starting Age in Children’s Developmental Milestones

It’s been an issue in doing this work to pin down the variability in the timing of the milestones. If a child has skills that seem like they are in Milestone 8, but the timing aligns with 7, which one are they in?

It’s hard for me to answer that question with just that information alone. To get an idea of how a child is progressing, I need their entire set of data of how they’ve been hitting all milestones, over the course of a few months. I need exact examples of what is meant to better answer the question.

I have gotten such robust data sets in order to better pin down what is going on, and I have analyzed them. Often times, there is a miscommunication between what I meant and how it was interpreted. In this case, I update my descriptions to communicate better. Sometimes, there is a slight nuance in difference in skill or behavior between one milestone to the next. It is always positive when this happens, because in taking a harder look, new observations about what kind of development is happening emerge. Sometimes I realize I made a mistake in how I documented the milestones. Perhaps I lumped behavior that spanned over 6 weeks in what should have been divided over 2 milestones. I may, thus, create an entire new milestone and sprinkle the new skills seen differently.

But, still: what is the variability among children in what age they start the milestones? I did a more thorough study of the issue and got some better answers.

To get a better idea of the natural variability in the start of the milestones, I did a study on my three children. I looked at both their behaviors and their head shape, down to the day, to get a more precise picture of when each started each milestone. My next chapter on head shape shows off some of the results of this, and how I used head shape to identify what milestone each was at, even though the timing can get off by as much as 18 days.

What I found is that my two boys hit the milestones nearly identically. My daughter varied from them. Therefore, I put together the following charts that compare “boy” and “girl.” The “boy” is actually two boys: my two sons. I don’t intend this to be authoritative on how boys versus girls hit the milestones. But in that it offers a comparison between two different types of children, it is useful to separate them by boy and girl, for now.

This is the comparison of the day the milestones started for the Three Year Old Milestones of boy versus girl:

Between boy and girl, the start hit pretty bang on for the first 3 milestones. Then at 4A, they get off. The girl hit the milestones later that the boy. How much later she hit them kept drifting. Sometimes it was a little later, such as 8 days. Sometimes it was a lot later, up to 18 days. Sometimes the difference between 2 milestones, say between 4B and 5, was a lot less for her than the boy. This brought her more into alignment with the boy than previously, though still later. The “Offset” column shows this. She gets 1 day off, then 7 more, then 2 more, for a total of 10 days off at 4B. Then there is less of a gap for her than the boy from 4B to 5, bringing her to be only 8 days different from her brothers at Milestone 5.

The milestones stay off until 8B, when they abruptly realign. They align so abruptly that I couldn’t tell a difference between 8A and 8B for the girl. There was a gap between 8A and 8B for the boys, but not the girl. I had to conclude that she went through these milestones at the same time—and it was an incredibly rough time for her. After this, until the end of age 3, the milestones can get off by about a week again, but this is fairly standard in the milestones.

Here is a chart of this over a timeline. I have icons for girl and boy here. I only plotted the first 6 months of the age of 3—and I split them up into 2 charts. This was only to give a more intuitive understanding of how the milestones might operate. I split up the first three months of age 3 (3.0 until 3.3) and the second three months (3.3 until 3.6) into two charts for readability:

I did the start of the intense period for this chart, so there are some difference in this chart and the previous table, which lists the start of the milestone itself. Nonetheless, this timeline chart is only meant to more intuitively show off how the milestones “drift.”

Again, it’s easy to see that the first 3 milestones hit bang on. They get off at 4A.  The gap between girl and boy gets more narrow or wide for each milestone as it goes along. Milestone 4A has a bigger gap in start day than Milestone 5, etc.

Here is the chart for the second three months of age 3, from 3.3 until 3.6, overlapping slightly with the previous chart:

At Milestone 8B, the timing of the starting age for the intense period of the milestone realigns suddenly. Notice that for the girl, 6B happens after the boy’s Milestone 7. Similarly, Milestone 7 for the girl happens after Milestone 8A for the boy. This handily explains why some might see skills at Milestone 7 when the timeline previously lined them up with 6B.

Given this new information, I restructured the timing of the milestones. Starting at 4A, until 8B, I describe the start of the milestones in terms of the previous milestone. For instance, Milestone 5 starts X days after 4B. I give a general age range of when that might be (e.g., 3.1.2 to 3.1.4) to help you get your bearing. But offsetting it from one milestone when pertinent hopefully gives better insight into how to follow the milestones.

Certainly, more study could be done. I did this original study to give some insight into how further study might be done and how the variability might operate. Assuming the milestones are really well-defined and follow in a predictable order (and this is a big assumption, I know), all it would take is more data sets with more children to get yet more insight into the variability across humans. And several have told me that in the late threes in particular, they see their child hit the milestones earlier than listed. It’s just how it goes. Human development is a vast, nuanced, awe-inspiring thing. As such, my recommendation is to follow the milestones more in numerical order than chronological. The timeline is meant to get you approximately to the right age.

But doing this admittedly small study, I hope, might give some insight into how natural variability operates. It’s not so terribly simple as children get one week off then stay one week off. They might get one week off, then get 10 days off, then only 6 days off, then totally realign.

And in doing this study, I do strongly recommend using the due date as your original date. Assume any deviation after that is due to natural variation in child development.

This will appear in my next book Misbehavior is Growth: 3 Year Olds

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