Depending on your definition, you can argue that a baby’s intelligence development starts in the womb. Around 2-3 weeks after fertilization, the baby’s neural plate forms which eventually develops into an actual brain.
This is the period of time when their brains grow the most. So much so, in fact, that at the time of birth, the vast majority of the baby’s brain is around 25% of its adult size — all in 9, or so, months.
The cerebral cortex (the part of the brain associated with voluntary actions) grows last, and is relatively underdeveloped at birth. However, it rapidly matures within the first fews years of a baby’s life.
By the age of 3, a child’s brain will be around 80% of its final adult size. From birth, and throughout their childhood, the neurons of the brain grow the fastest, until they peak at around the beginning of adolescence.
It is for this reason that babies (and toddlers) appear to be able to pick up new skills (like walking or talking) so rapidly — when compared to adults. These connections get stronger over time as new experiences, and repeated experiences, first build and then reinforce these neural connections.
Synapse connections form at a faster rate during this period than in any other time in your child’s life. Any unused synapses will disappear over time. With their brains literally swimming in new synaptic connections, your baby is set up to learn many new things, very quickly.
During these first few years, your baby has all the hardware it needs. Now all they need is some data — experiences.