認知老化是指個體的認知能力隨著年齡增長逐漸衰退的現象和過程。值得注意的是，並非所有認知能力都隨著年齡而下降。心理學研究人員通常將認知能力 (有時也稱為智力)大致分為兩類：流體智力和晶體智力。流體智力指的是存儲和操縱新信息的能力 (例如，感知能力，注意力，處理信息的速度，記憶能力，不依賴於先前學習和經驗的流體推理能力)。這些能力通常於個體先前的學習和經驗無關。相反，晶體智力指的是提取及使用先前學習和經驗所獲取信息的能力 (例如，專業知識，生活經驗，智慧，語言能力，依賴於先前學習和經驗的推理能力)。雖然流體智力類別中的認知能力通常受老化影響，但晶體智力類別中的認知能力通常隨著年齡的增長而保持相對穩定 (Craik & Bialystok, 2006)。認知老化可以根據程度或嚴重程度的差別分為正常衰老，輕度認知障礙和認知障礙症 (例如，阿爾茨海默氏病)。正常老化指那些不會明顯干擾個人日常生活的認知衰退。認知障礙症是一個通用術語，包括一系列會明顯降低個體日常活動能力的嚴重認知衰退。輕度認知障礙介於正常老化和認知障礙症之間 —— 它對個體的日常活動存在一定干擾，但還未達到認知障礙症的診斷標準。目前普遍的觀點是認知衰老無法逆轉或治愈，但可通過乾預或治療延遲或緩解 (Ames et al., 2017; Buffington et al., 2013; Burns & Iliffe, 2009)。
Cognitive aging refers to the phenomenon and process in which individuals’ cognitive abilities, in general, decline as they get older. Worthy of note, not all cognitive abilities decline with age. Cognitive abilities (also known as intelligence) are often roughly classified into two categories in psychological research: fluid intelligence and crystallized intelligence. Fluid intelligence refers to the ability to process, store, and manipulate new information that is independent of individuals’ prior learning and experience (e.g., sensory ability, attention, the speed of processing, memory ability, fluid reasoning ability, and other abilities which do not depend on prior learning and experience). Crystallized intelligence, in contrast, refers to the ability to retrieve and use information that has been acquired through prior learning and experience (e.g., expertise, life experience, wisdom, language ability, and other abilities which depend on prior learning and experience). Whilst cognitive abilities in the fluid-intelligence category typically are subjected to age-associated decline, those in the crystallized-intelligence category are often shown to preserve or even improve with age (Craik & Bialystok, 2006). The age-associated cognitive decline could vary in the degree or severity, according to which cognitive decline can be categorized as normal aging, mild cognitive decline, and dementia (e.g., Alzheimer’s disease). Normal aging refers to age-related changes that do not evidently interfere with individuals’ daily life. Dementia is a general term that describes a group of symptoms associated with severe cognitive declines that reduce one’s ability to perform daily activities. Mild cognitive impairment is the borderland between normal aging and dementia—it somewhat interferes with one’s daily activities but does not meet the diagnostic criteria of dementia yet. Cognitive aging is thought to be irreversible or incurable but may be delayed or alleviated via intervention or treatment (Ames et al., 2017; Buffington et al., 2013; Burns & Iliffe, 2009).