Same brain, but as we grow we develop and strengthen lots of connections in our brain ‘circuitry’. Children develop these at a fast rate, hence they learn very quickly. Sometimes when we get old, we can lose some of these connections meaning we forget stuff.
I have a few friends who I think are still stuck with their child’s brain even though they have the body of an adult! Sometimes a child’s brain is more active in different areas than an adults. Things like an active imagination are prevalent in children but they lack things like fear and problem solving skills that are more developed in adults. That’s why kids are fearless and can play with an imaginary friend for hours where as adults think more about the consequences of their actions and find it hard to really get in to a game of teddy bears tea party!