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The window narrows; it does not slam shut

Online, every developmental window looks like a cliff edge. Science separates critical periods from sensitive ones, and the far more common second kind leaves room to catch up.

What is the difference between a sensitive period and a critical period?

A critical period is a window that closes for good. In early childhood they are the exception. Depth perception using both eyes is the textbook case: Hubel and Wiesel showed in kittens that blocking one eye during a specific early stretch permanently changed how the visual brain responded. That is why misaligned eyes in children are best treated young.

A sensitive period is different. For a while, the brain is unusually ready for a certain kind of experience; afterward, learning is still possible, just more effortful. Speech sounds are the classic example. Patricia Kuhl’s research shows babies start out able to tell apart the sounds of any language, and by about ten months they have tuned in to the ones they hear most. For an adult learning a new language, the door didn’t lock. It got heavier.

I’ve watched this at home. Barely two, my daughters sang along to songs in a language they didn’t speak, sounding like natives. When they later returned to music for piano lessons, that ease was gone.

What does a sensitive window look like in the data?

When my team at Kinedu charted how children pick up each skill, nearly every curve had the same shape: a flat start where almost no one has it, a steep rise, then a slow approach to the top. That steep middle is the window.

These are Kinedu data, from an internal analysis of roughly 3.5 million children that has not yet been published with peer review. They show when a skill usually appears, not why, or what helped any particular child.

What matters for worried parents: after the steep part, the curves don’t hit a wall. They keep rising, slowly, for months. If windows were deadlines, we would see a stuck group at the bottom that never moved. We don’t. As I put it in Chapter 2: “Earlier is better, because the steep part is where it is easiest. Later is not impossible, because the curve is still going up.”

Is it too late after age 3?

No. The brain’s capacity to change is highest in the first years, but a figure Pat Levitt developed with the Harvard Center on the Developing Child shows it tapering gradually, not dropping off an edge. Change stays possible across life; it just gets more costly.

So notice the window open right now rather than grieving one you think has passed. If an expected skill still hasn’t appeared after several months, or your child stops doing something she could do, talk to your pediatrician. Early support tends to work best, which is a reason to ask, not to panic.

Related: Everything is everything and The range of normal is enormous. The full “why” behind windows, including the evidence on recovery after a hard start, is in Chapter 2 of Big Bang Baby. You can read the Introduction free.