A demographic group consisting of individuals who have reached an advanced age, typically classified as 65 years and older, often referred to as senior citizens. This category also includes subsets like 'Young Old' to indicate seniors who are relatively younger and active, generally in their 60s to mid-70s, and 'Baby Boomers,' referring to those born during the post-World War II baby boom (1946-1964). Although the term 'adolescents' typically refers to younger individuals, in this context, it could be used to discuss the transition of these individuals into senior citizenship in long-term studies. 'Age Groups' and 'People' are broad terms that encompass all individuals, including senior citizens.