The leading cause of memory loss between the ages of 15 and 25 is

Traumatic brain injury (TBI).

Explanation:

The TBI also called as intracranial injury it occurs when any injury is caused externally. The leading cause of memory loss between the ages of 15 and 25 is traumatic brain injury (TBI).

The head trauma can cause TBI. The road accident, falls and assaults are the possible causes for TBI. The effect of this injury depends upon the location, type and acuteness of the  injury.

it takes hundreds of years to become stable.

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which fault and or fold did you find most difficult to diagram