premise: The hippocampus region of the brain usually starts to shrink when we reach the age of 30. This contraction is held to be responsible for age-related memory loss, and the older we get the worse it becomes. The process may be reversible though, as the hippocampus is the only region of the brain in which neurons can grow. New research suggests that the secret to growing them is physical exercise, which raises the possibility of us working out to boost our brain power. Unfortunately, it is not just any sort of exercise that stimulates the growth of neurons and this explains why physically active people suffer the same memory loss as the more sedentary.
hypothesis: The intended audience of the passage is the physically inactive rather than the general population.