Why the First Five Years Matter (Without the Fear-Mongering)

Why early brain development is about building foundations, not sealing fate.

If you’ve ever heard someone say “90% of the brain develops by age five” and felt either panicked or skeptical then you’re not wrong to pause.

The first five years are critically important. But not because childhood is a one-shot window where everything is locked in forever. They matter because this is when the brain is building its basic architecture: wiring systems that later learning, behavior, and emotional health depend on.

Think of it less like pouring concrete and more like laying down the electrical grid of a city. You can renovate later, but early infrastructure shapes what’s easier, harder, or more fragile down the line.

Let’s unpack what’s really happening in those early years, using the science behind images that are commonly found in textbooks (and social media clickbait).

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Key Concepts

Synapses: Connections between neurons. More synapses = more potential communication pathways.

Synaptic pruning: The brain’s process of trimming unused or inefficient connections to make networks faster and more specialized.

Myelination: The insulation of neural pathways with myelin, which speeds up communication between brain regions.

Plasticity: The brain’s capacity to change in response to experience. Highest in early childhood, but never zero.

Prefrontal cortex: The brain region involved in planning, impulse control, attention, and complex decision-making. It develops slowly and last.

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Research Spotlight: What the Brain Is Actually Doing Early On

One of the most misunderstood facts about early brain development is this:

Early childhood is not about “finishing” the brain. It’s about overbuilding it.

Classic developmental neuroscience work (Thompson, 2001; Thompson & Nelson, 2001) shows that shortly after birth, the brain enters a period of rapid synapse formation, especially in sensory areas (vision, hearing), then language, and later higher-order cognitive systems.

Image from BioNinja

This surge is followed by experience-guided synaptic pruning which simply means that connections that are used repeatedly are strengthened and those that aren’t are eliminated (yes, our brains do use a ‘use it or lose it’ guideline).

Importantly:

• Sensory systems peak first (infancy)

• Language networks follow (toddlerhood → early childhood)

• Prefrontal systems grow and prune over a much longer timeline, extending into adolescence

Image from Sixpence Early Learning Fund

Myelination follows a similar pattern: fast early gains, continued refinement across childhood, and another acceleration during adolescence, especially in the prefrontal cortex.

While myelination begins early and accelerates in infancy, it continues to refine brain connections throughout childhood and adolescence, especially in brain regions responsible for planning, attention, and self-control. Image from Ouyang et al. (2019).

This is why early experiences don’t just teach skills but they actually help decide which neural pathways stay online.

Later work synthesizing neuroscience and developmental psychology reinforces this point: early caregiving, language exposure, and stress regulation shape how efficiently these systems organize, without determining outcomes in a rigid or deterministic way (Miller et al., 2012).

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Why This Matters

The takeaway is not that parents must optimize every moment. It is that early childhood environments are biologically influential because the brain is especially open to input.

What matters most during this window:

• Reliable relationships (not enrichment products)

• Language embedded in interaction, not passive exposure

• Manageable stress, buffered by responsive adults

• Opportunities to explore, move, and practice self-control gradually

The science consistently shows that ordinary, predictable experiences (e.g., being talked to, soothed, played with, and protected) are exactly what the developing brain expects — these are called experience-expectant processes).

This is also why early adversity can have outsized effects and why supportive interventions later in childhood still work. Plasticity narrows, but it never disappears.

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Long Story Short

The first 5 years matter because they’re when the brain is busiest building, testing, and refining its core systems. Early experiences don’t seal a child’s fate, but they do shape how sturdy and flexible the foundation is. Early childhood isn’t destiny. It’s infrastructure.

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Quick Takeaways

• The brain overproduces connections early, then prunes based on experience.

• Sensory and language systems develop before self-control and reasoning.

• The prefrontal cortex is nowhere near finished by age five.

• Relationships and everyday interactions matter more than “brain-boosting” products.

• Early support helps, but growth and change remain possible across development.

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Development Decoded is built on a simple belief: science should be clear, practical, and accessible to anyone who cares about kids. I write, research, and create resources outside of my day job and family life to make that happen. If you’d like to help fuel the work, you can now do so with one coffee at a time.

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References

• Miller, E. K., Maguire, L. K., & Macdonald, G. (2012). Home-based child development interventions for preschool children from socially disadvantaged families. Campbell Systematic Reviews, 1. doi:10.4073/csr.2012.1

• Ouyang, M., Dubois, J., Yu, Q., Mukherjee, P., & Huang, H. (2019). Delineation of early brain development from fetuses to infants with diffusion MRI and beyond. Neuroimage, 185, 836-850.

• Thompson, R. A. (2001). Development in the first years of life. The Future of Children, 11(1), 20-33. https://doi.org/10.2307/1602807

• Thompson, R. A., & Nelson, C. A. (2001). Developmental science and the media: Early brain development. American Psychologist, 56(1), 5.