Final answer:
The exact percentage of individuals who experience significant brain loss before the age of 65 is not provided in the available information. Generally, as people approach old age, there is a notable decrease in physiological functions, including a decline in the number of brain cells and increased incidence of cognitive impairments.
Step-by-step explanation:
Before the age of 65, it is not specified percent of individuals experience significant brain loss. Unfortunately, the information provided does not give a direct answer to this specific question. However, it is known that as people reach old age, which begins in the mid-60s, there is a general decline in various physiological functions, including brain function. During old age, the number of brain cells decreases, and diseases like Alzheimer's disease, which lead to the loss of mental function, become more prevalent. Nearly half of those over the age of 85 years show at least mild cognitive impairment according to health data.
The immune system's efficiency also declines, thus increasing the risk of serious illnesses such as cancer and pneumonia in the elderly. Although the agility of brain and body generally diminish with age, the exact percentage of individuals who experience significant brain loss before 65 is not clearly indicated in the given resources. For a precise percentage related to significant brain loss before the age of 65, it would be necessary to refer to additional scientific studies or medical resources that provide such specific data.