On the other hand, it also forestalls the symptoms of Alzheimer's.

My personal opinion is that kids' brains are still forming habits and that it should be treated like many families treat alcohol - "only for special occasions" or "just a taste". When a child demonstrates a reasonable level of emotional and intellectual responsibility, then they should be treated like an adult, and that includes regarding drug use. For some people that is age 14 and for others it isn't until they are 18 or 20.