After you've had a fine dinner with some friends, your memories of their faces, the taste of the food, and the music that was playing are distributed in the various visual, olfactory, and auditory areas of the brain, but they are all connected together by the hippocampus to form an "episode", rather than remaining a collection of separate memories. It seems to be the hippocampus that enables you to “play the scene back”, by reactivating this particular activity pattern in the various regions of the cortex.
But after a while, these various cortical regions activated during an event would become so strongly linked with one another that they would no longer need the hippocampus to act as their link. The memory of a piece of music that was playing that night could be enough to bring back the entire scene of the dinner party.
If we don't remember anything from our early childhood, like before our 4th birthday that's because hippocampus, part of the limbic system deep in the brain that stores long-term memories, had not yet matured. That doesn't mean earlier memories don't exist in our mind.
Here is one example: a woman was left by her father when she was just 2 years old. She can't consciously remember how he got drunk sometimes and abused her mother. But the emotions associated with the memory might be stored in her amygdala, another structure in the brain's limbic system that may be functional as early as birth. While highly emotional memories etched in the amygdala may not be accessible to the conscious mind, they might still influence the way we act and feel beyond our awareness.
The last area of the brain to reach maturity is the prefrontal cortex, where the so-called executive brain resides—where we make social judgments, weigh alternatives, plan for the future, and hold our behavior in check.
"The executive brain doesn't hit adult levels until the age of 25," says Jay Giedd of the National Institute of Mental Health, one of the lead scientists on the neuroimaging studies. "At puberty, you have adult passions, sex drive, energy, and emotion, but the reining in doesn't happen until much later." It is no wonder, that teenagers seem to lack good judgment or the ability to restrain impulses. "We can vote at 18," says Giedd, "and drive a car. But you can't rent a car until you're 25."