There is no universally agreed-upon definition of "middle age," as the age at which someone is considered to be in this stage of life can vary depending on cultural, societal, and personal factors. Some people might consider middle age to begin in their late 30s or early 40s, while others might consider it to begin in their 50s or even later.
It is important to note that the concept of middle age is subjective and can vary greatly from one individual to another. Some people may feel that they are still young and energetic in their 50s, while others may feel that they are already entering old age in their 40s. Ultimately, how one perceives and experiences middle age will depend on their own personal circumstances and outlook.