What is the average onset age of senility and dementia? https://www.quora.com/What-is-the-average-onset-age-of-senility-and-dementia/answer/K-Mustafa-2 Dementia is more common in people over the age of 65, but it can also affect younger people. Early onset of the disease can begin when people are in their 30s, 40s, or 50s. With treatment and early diagnosis, you can slow the progression of the disease and maintain mental function. https://www.verywellhealth.com/what-does-senile The most basic definition of senile from Merriam-Webster is "relating to, exhibiting, or characteristic of, old age." Thus, the pure use of "senile" simply refers to its age. However, the use of the word senile is more commonly, but somewhat incorrectly, associated with a decline in mental abilities, such as memory loss or confusion, as people age. Take, for example, this sentence: "Their senile grandmother would never remember their visits, but they knew they brightened her da...