Susannah Locke summarizes new research on the question:
The paper concludes that the new cells that are constantly being formed in very young brains may be messing up the circuits that hold memories. The brain makes new cells throughout life — a process called neurogenesis — but young people produce new neurons at a much higher rate. And this process is particularly active in the hippocampus, which deals with memories and learning. Most of the time, neurogenesis leads to better learning and improved memory. But there’s a catch. According to the Science paper, the extremely high rates of neurogenesis seen in very young children can actually increase forgetfulness. These new neurons could be crowding out the old circuits that hold memories.
Clare Wilson explains how the study was conducted:
[Katherine Akers] and her team taught mice of different ages to associate a particular environment with a mild electric shock. They then got some of the adult mice to run on a wheel, because this has been shown to promote the growth of new neurons.
When mice were placed back in the threatening environment, adult mice that had boosted their neuron numbers by running were less likely to freeze to the spot – a sure sign of fear – than a control group with no access to an exercise wheel.
This suggests that forming brain cells caused the mice to forget the electric shocks. Akers’s team then gave a group of mice just a few weeks old a drug that inhibits neurogenesis. These mice were more likely to remember the electric shock than a control group.