A young person's body is going through lots of changes during and following puberty and their brain is no exception. The brain is still developing into the early twenties.
Following puberty, the brain goes through a critical period of development. At this time the brain reorganises itself to become more efficient for functioning as an adult. To appreciate how the developing brain is affected by alcohol you need to understand what is happening to the brain at this time.
Studies of brain scans of children through childhood to the early twenties show the volume of the brain increases in size, then reduces again. This happens because the connections between different parts of the brain are altered through the teen years into your early 20's allowing the brain to adapt more easily to different situations.
As your brain matures it discards the unused pathways allowing the brain to be more efficient and able to perform the more complex forms of thought and behaviour that adults use.