Anewborn’s brain is approximately 25% of it’s adult weight, but a 5-year-old has a brain that is % of its adult size.

Approximately 95%

Explanation:

The first few years of a baby's life are the most crucial to its development. This is the time when it is adjusting to the world and learning to understand and interpret it, not to mention all the biological and chemical changes and developments that are going on internally.

When a baby's brain grows, its skull also grows to make room for it, this means that it has to grow quickly so that it can solidify the bone and form a solid protective structure from a young age, instead of growing up into adulthood with a skull that is still growing to make space for the brain.