Brain

It's a joke at times: People tend to become forgetful as they get older. 

But there is nothing funny about Alzheimer’s disease, an age-related brain condition that gradually destroys a person’s memory and thinking skills. It eventually prevents a person from being able to do simple tasks. Alzheimer’s disease progresses slowly, and the patient’s mental decline usually occurs in three classified stages: an early, preclinical stage with no symptoms, a middle stage with mild cognitive impairment and a final stage of Alzhimer’s dementia.